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  Table of contents
  Use multi-stage builds
  Multi-stage builds let you reduce the size of your final image, by creating a
  cleaner separation between the building of your image and the final output.
  Split your Dockerfile instructions into distinct stages to make sure that the
  resulting output only contains the files that are needed to run the application.
  Using multiple stages can also let you build more efficiently by executing
  build steps in parallel.
  See
  
  Multi-stage builds
   for more
  information.
  Create reusable stages
  If you have multiple images with a lot in common, consider creating a reusable
  stage that includes the shared components, and basing your unique stages on
  that. Docker only needs to build the common stage once. This means that your derivative images use memory
  on the Docker host more efficiently and load more quickly.
  It's also easier to maintain a common base stage ("Don't repeat yourself"),
  than it is to have multiple different stages doing similar things.
  Choose the right base image
  The first step towards achieving a secure image is to choose the right base
  image. When choosing an image, ensure it's built from a trusted source and keep
  it small.
  Docker Official Images
  
  are a curated collection that have clear documentation, promote best
  practices, and are regularly updated. They provide a trusted starting point
  for many applications.
  Verified Publisher
   images
  are high-quality images published and maintained by the organizations
  partnering with Docker, with Docker verifying the authenticity of the content
  in their repositories.
  Docker-Sponsored Open Source
  
  are published and maintained by open source projects sponsored by Docker
  through an 
  open source program
  .
  When you pick your base image, look out for the badges indicating that the
  image is part of these programs.
  
  When building your own image from a Dockerfile, ensure you choose a minimal base
  image that matches your requirements. A smaller base image not only offers
  portability and fast downloads, but also shrinks the size of your image and
  minimizes the number of vulnerabilities introduced through the dependencies.
  You should also consider using two types of base image: one for building and
  unit testing, and another (typically slimmer) image for production. In the
  later stages of development, your image may not require build tools such as
  compilers, build systems, and debugging tools. A small image with minimal
  dependencies can considerably lower the attack surface.
  Rebuild your images often
  Docker images are immutable. Building an image is taking a snapshot of that
  image at that moment. That includes any base images, libraries, or other
  software you use in your build. To keep your images up-to-date and secure, make
  sure to rebuild your image often, with updated dependencies.
  To ensure that you're getting the latest versions of dependencies in your build,
  you can use the 
  --no-cache
   option to avoid cache hits.
  
  $
   docker build --no-cache -t my-image:my-tag .
  
  The following Dockerfile uses the 
  24.04
   tag of the 
  ubuntu
   image. Over time,
  that tag may resolve to a different underlying version of the 
  ubuntu
   image,
  as the publisher rebuilds the image with new security patches and updated
  libraries. Using the 
  --no-cache
  , you can avoid cache hits and ensure a fresh
  download of base images and dependencies.
  
  # syntax=docker/dockerfile:1
  
  FROM
   ubuntu:24.04
  
  RUN
   apt-get -y update 
  &&
   apt-get install -y --no-install-recommends python3
  Also consider 
  pinning base image versions
  .
  Exclude with .dockerignore
  To exclude files not relevant to the build, without restructuring your source
  repository, use a 
  .dockerignore
   file. This file supports exclusion patterns
  similar to 
  .gitignore
   files.
  For example, to exclude all files with the 
  .md
   extension:
  
  *.md
  For information on creating one, see
  
  Dockerignore file
  .
  Create ephemeral containers
  The image defined by your Dockerfile should generate containers that are as
  ephemeral as possible. Ephemeral means that the container can be stopped
  and destroyed, then rebuilt and replaced with an absolute minimum set up and
  configuration.
  Refer to 
  Processes
   under 
  The Twelve-factor App
  
  methodology to get a feel for the motivations of running containers in such a
  stateless fashion.
  Don't install unnecessary packages
  Avoid installing extra or unnecessary packages just because they might be nice to have. For example, you don’t need to include a text editor in a database image.
  When you avoid installing extra or unnecessary packages, your images have reduced complexity, reduced dependencies, reduced file sizes, and reduced build times.
  Decouple applications
  Each container should have only one concern. Decoupling applications into
  multiple containers makes it easier to scale horizontally and reuse containers.
  For instance, a web application stack might consist of three separate
  containers, each with its own unique image, to manage the web application,
  database, and an in-memory cache in a decoupled manner.
  Limiting each container to one process is a good rule of thumb, but it's not a
  hard and fast rule. For example, not only can containers be
  
  spawned with an init process
  ,
  some programs might spawn additional processes of their own accord. For
  instance, 
  Celery
   can spawn multiple worker
  processes, and 
  Apache
   can create one process per
  request.
  Use your best judgment to keep containers as clean and modular as possible. If
  containers depend on each other, you can use
  
  Docker container networks
  
  to ensure that these containers can communicate.
  Sort multi-line arguments
  Whenever possible, sort multi-line arguments alphanumerically to make maintenance easier.
  This helps to avoid duplication of packages and make the
  list much easier to update. This also makes PRs a lot easier to read and
  review. Adding a space before a backslash (
  \
  ) helps as well.
  Here’s an example from the 
  buildpack-deps image
  :
  
  RUN
   apt-get update 
  &&
   apt-get install -y --no-install-recommends 
  \
  
    bzr 
  \
  
    cvs 
  \
  
    git 
  \
  
    mercurial 
  \
  
    subversion 
  \
  
    
  &&
   rm -rf /var/lib/apt/lists/*
  Leverage build cache
  When building an image, Docker steps through the instructions in your
  Dockerfile, executing each in the order specified. For each instruction, Docker
  checks whether it can reuse the instruction from the build cache.
  Understanding how the build cache works, and how cache invalidation occurs,
  is critical for ensuring faster builds.
  For more information about the Docker build cache and how to optimize your builds,
  see
  
  Docker build cache
  .
  Pin base image versions
  Image tags are mutable, meaning a publisher can update a tag to point to a new
  image. This is useful because it lets publishers update tags to point to
  newer versions of an image. And as an image consumer, it means you
  automatically get the new version when you re-build your image.
  For example, if you specify 
  FROM alpine:3.21
   in your Dockerfile, 
  3.21
  
  resolves to the latest patch version for 
  3.21
  .
  
  # syntax=docker/dockerfile:1
  
  FROM
   alpine:3.21
  At one point in time, the 
  3.21
   tag might point to version 3.21.1 of the
  image. If you rebuild the image 3 months later, the same tag might point to a
  different version, such as 3.21.4. This publishing workflow is best practice,
  and most publishers use this tagging strategy, but it isn't enforced.
  The downside with this is that you're not guaranteed to get the same for every
  build. This could result in breaking changes, and it means you also don't have
  an audit trail of the exact image versions that you're using.
  To fully secure your supply chain integrity, you can pin the image version to a
  specific digest. By pinning your images to a digest, you're guaranteed to
  always use the same image version, even if a publisher replaces the tag with a
  new image. For example, the following Dockerfile pins the Alpine image to the
  same tag as earlier, 
  3.21
  , but this time with a digest reference as well.
  
  # syntax=docker/dockerfile:1
  
  FROM
   alpine:3.21@sha256:a8560b36e8b8210634f77d9f7f9efd7ffa463e380b75e2e74aff4511df3ef88c
  With this Dockerfile, even if the publisher updates the 
  3.21
   tag, your builds
  would still use the pinned image version:
  
  a8560b36e8b8210634f77d9f7f9efd7ffa463e380b75e2e74aff4511df3ef88c
  .
  While this helps you avoid unexpected changes, it's also more tedious to have
  to look up and include the image digest for base image versions manually each
  time you want to update it. And you're opting out of automated security fixes,
  which is likely something you want to get.
  Docker Scout's default 
  Up-to-Date Base Images
  
  policy
   checks whether the
  base image version you're using is in fact the latest version. This policy also
  checks if pinned digests in your Dockerfile correspond to the correct version.
  If a publisher updates an image that you've pinned, the policy evaluation
  returns a non-compliant status, indicating that you should update your image.
  Docker Scout also supports an automated remediation workflow for keeping your
  base images up-to-date. When a new image digest is available, Docker Scout can
  automatically raise a pull request on your repository to update your
  Dockerfiles to use the latest version. This is better than using a tag that
  changes the version automatically, because you're in control and you have an
  audit trail of when and how the change occurred.
  For more information about automatically updating your base images with Docker
  Scout, see
  
  Remediation
  .
  Build and test your images in CI
  When you check in a change to source control or create a pull request, use
  
  GitHub Actions
   or another CI/CD pipeline to
  automatically build and tag a Docker image and test it.
  Dockerfile instructions
  Follow these recommendations on how to properly use the
  
  Dockerfile instructions
  
  to create an efficient and maintainable Dockerfile.
  
  Tip
  To improve linting, code navigation, and vulnerability scanning of your Dockerfiles in Visual Studio Code
  see 
  Docker VS Code Extension
  .
  FROM
  Whenever possible, use current official images as the basis for your
  images. Docker recommends the 
  Alpine image
   as it
  is tightly controlled and small in size (currently under 6 MB), while still
  being a full Linux distribution.
  For more information about the 
  FROM
   instruction, see
  
  Dockerfile reference for the FROM instruction
  .
  LABEL
  You can add labels to your image to help organize images by project, record
  licensing information, to aid in automation, or for other reasons. For each
  label, add a line beginning with 
  LABEL
   with one or more key-value pairs.
  The following examples show the different acceptable formats. Explanatory comments are included inline.
  Strings with spaces must be quoted or the spaces must be escaped. Inner
  quote characters (
  "
  ), must also be escaped. For example:
  
  # Set one or more individual labels
  
  LABEL
   com.example.version
  =
  "0.0.1-beta"
  
  LABEL
   
  vendor1
  =
  "ACME Incorporated"
  
  LABEL
   
  vendor2
  =
  ZENITH
  \ 
  Incorporated
  
  LABEL
   com.example.release-date
  =
  "2015-02-12"
  
  LABEL
   com.example.version.is-production
  =
  ""
  An image can have more than one label. Prior to Docker 1.10, it was recommended
  to combine all labels into a single 
  LABEL
   instruction, to prevent extra layers
  from being created. This is no longer necessary, but combining labels is still
  supported. For example:
  
  # Set multiple labels on one line
  
  LABEL
   com.example.version
  =
  "0.0.1-beta"
   com.example.release-date
  =
  "2015-02-12"
  The above example can also be written as:
  
  # Set multiple labels at once, using line-continuation characters to break long lines
  
  LABEL
   
  vendor
  =
  ACME
  \ 
  Incorporated 
  \
  
        com.example.is-beta
  =
   
  \
  
        com.example.is-production
  =
  ""
   
  \
  
        com.example.version
  =
  "0.0.1-beta"
   
  \
  
        com.example.release-date
  =
  "2015-02-12"
  See
  
  Understanding object labels
  
  for guidelines about acceptable label keys and values. For information about
  querying labels, refer to the items related to filtering in
  
  Managing labels on objects
  .
  See also
  
  LABEL
   in the Dockerfile reference.
  RUN
  Split long or complex 
  RUN
   statements on multiple lines separated with
  backslashes to make your Dockerfile more readable, understandable, and
  maintainable.
  For example, you can chain commands with the 
  &&
   operator, and use
  escape characters to break long commands into multiple lines.
  
  RUN
   apt-get update 
  &&
   apt-get install -y --no-install-recommends 
  \
  
      package-bar 
  \
  
      package-baz 
  \
  
      package-foo
  By default, backslash escapes a newline character, but you can change it with
  the
  
  escape
   directive
  .
  You can also use here documents to run multiple commands without chaining them
  with a pipeline operator:
  
  RUN
   <<EOF
  
  apt-get update
  
  apt-get install -y --no-install-recommends 
  \
  
      package-bar 
  \
  
      package-baz 
  \
  
      package-foo
  
  EOF
  For more information about 
  RUN
  , see
  
  Dockerfile reference for the RUN instruction
  .
  apt-get
  One common use case for 
  RUN
   instructions in Debian-based images is to install
  software using 
  apt-get
  . Because 
  apt-get
   installs packages, the 
  RUN apt-get
   command has several counter-intuitive behaviors to look out for.
  Always combine 
  RUN apt-get update
   with 
  apt-get install
   in the same 
  RUN
  
  statement. For example:
  
  RUN
   apt-get update 
  &&
   apt-get install -y --no-install-recommends 
  \
  
      package-bar 
  \
  
      package-baz 
  \
  
      package-foo
  Using 
  apt-get update
   alone in a 
  RUN
   statement causes caching issues and
  subsequent 
  apt-get install
   instructions to fail. For example, this issue will occur in the following Dockerfile:
  
  # syntax=docker/dockerfile:1
  
  FROM
   ubuntu:22.04
  
  RUN
   apt-get update
  
  RUN
   apt-get install -y --no-install-recommends curl
  After building the image, all layers are in the Docker cache. Suppose you later
  modify 
  apt-get install
   by adding an extra package as shown in the following Dockerfile:
  
  # syntax=docker/dockerfile:1
  
  FROM
   ubuntu:22.04
  
  RUN
   apt-get update
  
  RUN
   apt-get install -y --no-install-recommends curl nginx
  Docker sees the initial and modified instructions as identical and reuses the
  cache from previous steps. As a result the 
  apt-get update
   isn't executed
  because the build uses the cached version. Because the 
  apt-get update
   isn't
  run, your build can potentially get an outdated version of the 
  curl
   and
  
  nginx
   packages.
  Using 
  RUN apt-get update && apt-get install -y --no-install-recommends
   ensures your Dockerfile
  installs the latest package versions with no further coding or manual
  intervention. This technique is known as cache busting. You can also achieve
  cache busting by specifying a package version. This is known as version pinning.
  For example:
  
  RUN
   apt-get update 
  &&
   apt-get install -y --no-install-recommends 
  \
  
      package-bar 
  \
  
      package-baz 
  \
  
      package-foo
  =
  1.3.*
  Version pinning forces the build to retrieve a particular version regardless of
  what’s in the cache. This technique can also reduce failures due to unanticipated changes
  in required packages.
  Below is a well-formed 
  RUN
   instruction that demonstrates all the 
  apt-get
  
  recommendations.
  
  RUN
   apt-get update 
  &&
   apt-get install -y --no-install-recommends 
  \
  
      aufs-tools 
  \
  
      automake 
  \
  
      build-essential 
  \
  
      curl 
  \
  
      dpkg-sig 
  \
  
      libcap-dev 
  \
  
      libsqlite3-dev 
  \
  
      mercurial 
  \
  
      reprepro 
  \
  
      ruby1.9.1 
  \
  
      ruby1.9.1-dev 
  \
  
      
  s3cmd
  =
  1.1.* 
  \
  
      
  &&
   rm -rf /var/lib/apt/lists/*
  The 
  s3cmd
   argument specifies a version 
  1.1.*
  . If the image previously
  used an older version, specifying the new one causes a cache bust of 
  apt-get update
   and ensures the installation of the new version. Listing packages on
  each line can also prevent mistakes in package duplication.
  In addition, when you clean up the apt cache by removing 
  /var/lib/apt/lists
   it
  reduces the image size, since the apt cache isn't stored in a layer. Since the
  
  RUN
   statement starts with 
  apt-get update
  , the package cache is always
  refreshed prior to 
  apt-get install
  .
  Official Debian and Ubuntu images 
  automatically run 
  apt-get clean
  , so explicit invocation is not required.
  Using pipes
  Some 
  RUN
   commands depend on the ability to pipe the output of one command into another, using the pipe character (
  |
  ), as in the following example:
  
  RUN
   wget -O - https://some.site 
  |
   wc -l > /number
  Docker executes these commands using the 
  /bin/sh -c
   interpreter, which only
  evaluates the exit code of the last operation in the pipe to determine success.
  In the example above, this build step succeeds and produces a new image so long
  as the 
  wc -l
   command succeeds, even if the 
  wget
   command fails.
  If you want the command to fail due to an error at any stage in the pipe,
  prepend 
  set -o pipefail &&
   to ensure that an unexpected error prevents the
  build from inadvertently succeeding. For example:
  
  RUN
   
  set
   -o pipefail 
  &&
   wget -O - https://some.site 
  |
   wc -l > /number
  
  Note
  Not all shells support the 
  -o pipefail
   option.
  In cases such as the 
  dash
   shell on
  Debian-based images, consider using the 
  exec
   form of 
  RUN
   to explicitly
  choose a shell that does support the 
  pipefail
   option. For example:
  
  RUN
   
  [
  "/bin/bash"
  , 
  "-c"
  , 
  "set -o pipefail && wget -O - https://some.site | wc -l > /number"
  ]
  CMD
  The 
  CMD
   instruction should be used to run the software contained in your
  image, along with any arguments. 
  CMD
   should almost always be used in the form
  of 
  CMD ["executable", "param1", "param2"]
  . Thus, if the image is for a
  service, such as Apache and Rails, you would run something like 
  CMD ["apache2","-DFOREGROUND"]
  . Indeed, this form of the instruction is recommended
  for any service-based image.
  In most other cases, 
  CMD
   should be given an interactive shell, such as bash,
  Python and perl. For example, 
  CMD ["perl", "-de0"]
  , 
  CMD ["python"]
  , or 
  CMD ["php", "-a"]
  . Using this form means that when you execute something like
  
  docker run -it python
  , you’ll get dropped into a usable shell, ready to go.
  
  CMD
   should rarely be used in the manner of 
  CMD ["param", "param"]
   in
  conjunction with
  
  ENTRYPOINT
  , unless
  you and your expected users are already quite familiar with how 
  ENTRYPOINT
  
  works.
  For more information about 
  CMD
  , see
  
  Dockerfile reference for the CMD instruction
  .
  EXPOSE
  The 
  EXPOSE
   instruction indicates the ports on which a container listens
  for connections. Consequently, you should use the common, traditional port for
  your application. For example, an image containing the Apache web server would
  use 
  EXPOSE 80
  , while an image containing MongoDB would use 
  EXPOSE 27017
   and
  so on.
  For external access, your users can execute 
  docker run
   with a flag indicating
  how to map the specified port to the port of their choice.
  For container linking, Docker provides environment variables for the path from
  the recipient container back to the source (for example, 
  MYSQL_PORT_3306_TCP
  ).
  For more information about 
  EXPOSE
  , see
  
  Dockerfile reference for the EXPOSE instruction
  .
  ENV
  To make new software easier to run, you can use 
  ENV
   to update the
  
  PATH
   environment variable for the software your container installs. For
  example, 
  ENV PATH=/usr/local/nginx/bin:$PATH
   ensures that 
  CMD ["nginx"]
  
  just works.
  The 
  ENV
   instruction is also useful for providing the required environment
  variables specific to services you want to containerize, such as Postgres’s
  
  PGDATA
  .
  Lastly, 
  ENV
   can also be used to set commonly used version numbers so that
  version bumps are easier to maintain, as seen in the following example:
  
  ENV
   
  PG_MAJOR
  =
  9
  .3
  
  ENV
   
  PG_VERSION
  =
  9
  .3.4
  
  RUN
   curl -SL https://example.com/postgres-
  $PG_VERSION
  .tar.xz 
  |
   tar -xJC /usr/src/postgres 
  &&
   …
  
  ENV
   
  PATH
  =
  /usr/local/postgres-
  $PG_MAJOR
  /bin:
  $PATH
  Similar to having constant variables in a program, as opposed to hard-coding
  values, this approach lets you change a single 
  ENV
   instruction to
  automatically bump the version of the software in your container.
  Each 
  ENV
   line creates a new intermediate layer, just like 
  RUN
   commands. This
  means that even if you unset the environment variable in a future layer, it
  still persists in this layer and its value can be dumped. You can test this by
  creating a Dockerfile like the following, and then building it.
  
  # syntax=docker/dockerfile:1
  
  FROM
   alpine
  
  ENV
   
  ADMIN_USER
  =
  "mark"
  
  RUN
   
  echo
   
  $ADMIN_USER
   > ./mark
  
  RUN
   
  unset
   ADMIN_USER
  
  $
   docker run --rm 
  test
   sh -c 
  'echo $ADMIN_USER'
  
  mark
  
  To prevent this and unset the environment variable, use a 
  RUN
   command
  with shell commands, to set, use, and unset the variable all in a single layer.
  You can separate your commands with 
  ;
   or 
  &&
  . If you use the second method,
  and one of the commands fails, the 
  docker build
   also fails. This is usually a
  good idea. Using 
  \
   as a line continuation character for Linux Dockerfiles
  improves readability. You could also put all of the commands into a shell script
  and have the 
  RUN
   command just run that shell script.
  
  # syntax=docker/dockerfile:1
  
  FROM
   alpine
  
  RUN
   
  export
   
  ADMIN_USER
  =
  "mark"
   
  \
  
      
  &&
   
  echo
   
  $ADMIN_USER
   > ./mark 
  \
  
      
  &&
   
  unset
   ADMIN_USER
  
  CMD
   sh
  
  $
   docker run --rm 
  test
   sh -c 
  'echo $ADMIN_USER'
  
  For more information about 
  ENV
  , see
  
  Dockerfile reference for the ENV instruction
  .
  ADD or COPY
  ADD
   and 
  COPY
   are functionally similar. 
  COPY
   supports basic copying of
  files into the container, from the
  
  build context
  
  or from a stage in a
  
  multi-stage build
  .
  
  ADD
   supports features for fetching files from remote HTTPS and Git URLs, and
  extracting tar files automatically when adding files from the build context.
  You'll mostly want to use 
  COPY
   for copying files from one stage to another in
  a multi-stage build. If you need to add files from the build context to the
  container temporarily to execute a 
  RUN
   instruction, you can often substitute
  the 
  COPY
   instruction with a bind mount instead. For example, to temporarily
  add a 
  requirements.txt
   file for a 
  RUN pip install
   instruction:
  
  RUN
   --mount
  =
  type
  =
  bind,source
  =
  requirements.txt,target
  =
  /tmp/requirements.txt 
  \
  
      pip install --requirement /tmp/requirements.txt
  Bind mounts are more efficient than 
  COPY
   for including files from the build
  context in the container. Note that bind-mounted files are only added
  temporarily for a single 
  RUN
   instruction, and don't persist in the final
  image. If you need to include files from the build context in the final image,
  use 
  COPY
  .
  The 
  ADD
   instruction is best for when you need to download a remote artifact
  as part of your build. 
  ADD
   is better than manually adding files using
  something like 
  wget
   and 
  tar
  , because it ensures a more precise build cache.
  
  ADD
   also has built-in support for checksum validation of the remote
  resources, and a protocol for parsing branches, tags, and subdirectories from
  
  Git URLs
  .
  The following example uses 
  ADD
   to download a .NET installer. Combined with
  multi-stage builds, only the .NET runtime remains in the final stage, no
  intermediate files.
  
  # syntax=docker/dockerfile:1
  
  FROM
   scratch AS src
  
  ARG
   
  DOTNET_VERSION
  =
  8
  .0.0-preview.6.23329.7
  
  ADD
   --checksum
  =
  sha256:270d731bd08040c6a3228115de1f74b91cf441c584139ff8f8f6503447cebdbb 
  \
  
      https://dotnetcli.azureedge.net/dotnet/Runtime/
  $DOTNET_VERSION
  /dotnet-runtime-
  $DOTNET_VERSION
  -linux-arm64.tar.gz /dotnet.tar.gz
  
  FROM
   mcr.microsoft.com/dotnet/runtime-deps:8.0.0-preview.6-bookworm-slim-arm64v8 AS installer
  
  # Retrieve .NET Runtime
  
  RUN
   --mount
  =
  from
  =
  src,target
  =
  /src <<EOF
  
  mkdir -p /dotnet
  
  tar -oxzf /src/dotnet.tar.gz -C /dotnet
  
  EOF
  
  FROM
   mcr.microsoft.com/dotnet/runtime-deps:8.0.0-preview.6-bookworm-slim-arm64v8
  
  COPY
   --from
  =
  installer /dotnet /usr/share/dotnet
  
  RUN
   ln -s /usr/share/dotnet/dotnet /usr/bin/dotnet
  For more information about 
  ADD
   or 
  COPY
  , see the following:
  Dockerfile reference for the ADD instruction
  Dockerfile reference for the COPY instruction
  ENTRYPOINT
  The best use for 
  ENTRYPOINT
   is to set the image's main command, allowing that
  image to be run as though it was that command, and then use 
  CMD
   as the
  default flags.
  The following is an example of an image for the command line tool 
  s3cmd
  :
  
  ENTRYPOINT
   
  [
  "s3cmd"
  ]
  
  CMD
   
  [
  "--help"
  ]
  You can use the following command to run the image and show the command's help:
  
  $
   docker run s3cmd
  
  Or, you can use the right parameters to execute a command, like in the following example:
  
  $
   docker run s3cmd ls s3://mybucket
  
  This is useful because the image name can double as a reference to the binary as
  shown in the command above.
  The 
  ENTRYPOINT
   instruction can also be used in combination with a helper
  script, allowing it to function in a similar way to the command above, even
  when starting the tool may require more than one step.
  For example, the 
  Postgres Official Image
  
  uses the following script as its 
  ENTRYPOINT
  :
  
  #!/bin/bash
  
  set
   -e
  
  if
   
  [
   
  "
  $1
  "
   
  =
   
  'postgres'
   
  ]
  ;
   
  then
  
      chown -R postgres 
  "
  $PGDATA
  "
  
      
  if
   
  [
   -z 
  "
  $(
  ls -A 
  "
  $PGDATA
  "
  )
  "
   
  ]
  ;
   
  then
  
          gosu postgres initdb
  
      
  fi
  
      
  exec
   gosu postgres 
  "
  $@
  "
  
  fi
  
  exec
   
  "
  $@
  "
  This script uses 
  the 
  exec
   Bash command
   so that the final running application becomes the container's PID 1. This allows the application to receive any Unix signals sent to the container. For more information, see the
  
  ENTRYPOINT
   reference
  .
  In the following example, a helper script is copied into the container and run via 
  ENTRYPOINT
   on
  container start:
  
  COPY
   ./docker-entrypoint.sh /
  
  ENTRYPOINT
   
  [
  "/docker-entrypoint.sh"
  ]
  
  CMD
   
  [
  "postgres"
  ]
  This script lets you interact with Postgres in several ways.
  It can simply start Postgres:
  
  $
   docker run postgres
  
  Or, you can use it to run Postgres and pass parameters to the server:
  
  $
   docker run postgres postgres --help
  
  Lastly, you can use it to start a totally different tool, such as Bash:
  
  $
   docker run --rm -it postgres bash
  
  For more information about 
  ENTRYPOINT
  , see
  
  Dockerfile reference for the ENTRYPOINT instruction
  .
  VOLUME
  You should use the 
  VOLUME
   instruction to expose any database storage area,
  configuration storage, or files and folders created by your Docker container. You
  are strongly encouraged to use 
  VOLUME
   for any combination of mutable or user-serviceable
  parts of your image.
  For more information about 
  VOLUME
  , see
  
  Dockerfile reference for the VOLUME instruction
  .
  USER
  If a service can run without privileges, use 
  USER
   to change to a non-root
  user. Start by creating the user and group in the Dockerfile with something
  like the following example:
  
  RUN
   groupadd -r postgres 
  &&
   useradd --no-log-init -r -g postgres postgres
  
  Note
  Consider an explicit UID/GID.
  Users and groups in an image are assigned a non-deterministic UID/GID in that
  the "next" UID/GID is assigned regardless of image rebuilds. So, if it’s
  critical, you should assign an explicit UID/GID.
  
  Note
  Due to an 
  unresolved bug
   in the
  Go archive/tar package's handling of sparse files, attempting to create a user
  with a significantly large UID inside a Docker container can lead to disk
  exhaustion because 
  /var/log/faillog
   in the container layer is filled with
  NULL (\0) characters. A workaround is to pass the 
  --no-log-init
   flag to
  
  useradd
  . The Debian/Ubuntu 
  adduser
   wrapper does not support this flag.
  Avoid installing or using 
  sudo
   as it has unpredictable TTY and
  signal-forwarding behavior that can cause problems. If you absolutely need
  functionality similar to 
  sudo
  , such as initializing the daemon as 
  root
   but
  running it as non-
  root
  , consider using 
  “gosu”
  .
  Lastly, to reduce layers and complexity, avoid switching 
  USER
   back and forth
  frequently.
  For more information about 
  USER
  , see
  
  Dockerfile reference for the USER instruction
  .
  WORKDIR
  For clarity and reliability, you should always use absolute paths for your
  
  WORKDIR
  . Also, you should use 
  WORKDIR
   instead of proliferating instructions
  like 
  RUN cd … && do-something
  , which are hard to read, troubleshoot, and
  maintain.
  For more information about 
  WORKDIR
  , see
  
  Dockerfile reference for the 
  WORKDIR
   instruction
  .
  ONBUILD
  An 
  ONBUILD
   command executes after the current Dockerfile build completes.
  
  ONBUILD
   executes in any child image derived 
  FROM
   the current image. Think
  of the 
  ONBUILD
   command as an instruction that the parent Dockerfile gives
  to the child Dockerfile.
  A Docker build executes 
  ONBUILD
   commands before any command in a child
  Dockerfile.
  ONBUILD
   is useful for images that are going to be built 
  FROM
   a given
  image. For example, you would use 
  ONBUILD
   for a language stack image that
  builds arbitrary user software written in that language within the
  Dockerfile, as you can see in 
  Ruby’s 
  ONBUILD
   variants
  .
  Images built with 
  ONBUILD
   should get a separate tag. For example,
  
  ruby:1.9-onbuild
   or 
  ruby:2.0-onbuild
  .
  Be careful when putting 
  ADD
   or 
  COPY
   in 
  ONBUILD
  . The `onbuild image
  fails catastrophically if the new build's context is missing the resource being
  added. Adding a separate tag, as recommended above, helps mitigate this by
  allowing the Dockerfile author to make a choice.
  For more information about 
  ONBUILD
  , see
  
  Dockerfile reference for the ONBUILD instruction
  .
  
  ## Code Examples
  
  ```dockerfile
  # syntax=docker/dockerfile:1
  FROM ubuntu:24.04
  RUN apt-get -y update && apt-get install -y --no-install-recommends python3
  ```
  
  ```dockerfile
  # syntax=docker/dockerfile:1
  FROM ubuntu:24.04
  RUN apt-get -y update && apt-get install -y --no-install-recommends python3
  ```
  
  ```text
  RUN apt-get update && apt-get install -y --no-install-recommends \
    bzr \
    cvs \
    git \
    mercurial \
    subversion \
    && rm -rf /var/lib/apt/lists/*
  ```
  
  ```text
  RUN apt-get update && apt-get install -y --no-install-recommends \
    bzr \
    cvs \
    git \
    mercurial \
    subversion \
    && rm -rf /var/lib/apt/lists/*
  ```
  
  ```text
  # syntax=docker/dockerfile:1
  FROM alpine:3.21@sha256:a8560b36e8b8210634f77d9f7f9efd7ffa463e380b75e2e74aff4511df3ef88c
  ```
  
  ```text
  # syntax=docker/dockerfile:1
  FROM alpine:3.21@sha256:a8560b36e8b8210634f77d9f7f9efd7ffa463e380b75e2e74aff4511df3ef88c
  ```
  
  ```text
  a8560b36e8b8210634f77d9f7f9efd7ffa463e380b75e2e74aff4511df3ef88c
  ```
  
  ```text
  # Set one or more individual labels
  LABEL com.example.version="0.0.1-beta"
  LABEL vendor1="ACME Incorporated"
  LABEL vendor2=ZENITH\ Incorporated
  LABEL com.example.release-date="2015-02-12"
  LABEL com.example.version.is-production=""
  ```
  
  ```text
  # Set one or more individual labels
  LABEL com.example.version="0.0.1-beta"
  LABEL vendor1="ACME Incorporated"
  LABEL vendor2=ZENITH\ Incorporated
  LABEL com.example.release-date="2015-02-12"
  LABEL com.example.version.is-production=""
  ```
  
  ```text
  # Set multiple labels on one line
  LABEL com.example.version="0.0.1-beta" com.example.release-date="2015-02-12"
  ```
  
  ```text
  # Set multiple labels on one line
  LABEL com.example.version="0.0.1-beta" com.example.release-date="2015-02-12"
  ```
  
  ```text
  # Set multiple labels at once, using line-continuation characters to break long lines
  LABEL vendor=ACME\ Incorporated \
        com.example.is-beta= \
        com.example.is-production="" \
        com.example.version="0.0.1-beta" \
        com.example.release-date="2015-02-12"
  ```
  
  ```text
  # Set multiple labels at once, using line-continuation characters to break long lines
  LABEL vendor=ACME\ Incorporated \
        com.example.is-beta= \
        com.example.is-production="" \
        com.example.version="0.0.1-beta" \
        com.example.release-date="2015-02-12"
  ```
  
  ```text
  RUN apt-get update && apt-get install -y --no-install-recommends \
      package-bar \
      package-baz \
      package-foo
  ```
  
  ```text
  RUN apt-get update && apt-get install -y --no-install-recommends \
      package-bar \
      package-baz \
      package-foo
  ```
  
  ```text
  RUN <<EOF
  apt-get update
  apt-get install -y --no-install-recommends \
      package-bar \
      package-baz \
      package-foo
  EOF
  ```
  
  ```text
  RUN <<EOF
  apt-get update
  apt-get install -y --no-install-recommends \
      package-bar \
      package-baz \
      package-foo
  EOF
  ```
  
  ```text
  RUN apt-get update && apt-get install -y --no-install-recommends \
      package-bar \
      package-baz \
      package-foo
  ```
  
  ```text
  RUN apt-get update && apt-get install -y --no-install-recommends \
      package-bar \
      package-baz \
      package-foo
  ```
  
  ```dockerfile
  # syntax=docker/dockerfile:1
  
  FROM ubuntu:22.04
  RUN apt-get update
  RUN apt-get install -y --no-install-recommends curl
  ```
  
  ```dockerfile
  # syntax=docker/dockerfile:1
  
  FROM ubuntu:22.04
  RUN apt-get update
  RUN apt-get install -y --no-install-recommends curl
  ```
  
  ```dockerfile
  # syntax=docker/dockerfile:1
  
  FROM ubuntu:22.04
  RUN apt-get update
  RUN apt-get install -y --no-install-recommends curl nginx
  ```
  
  ```dockerfile
  # syntax=docker/dockerfile:1
  
  FROM ubuntu:22.04
  RUN apt-get update
  RUN apt-get install -y --no-install-recommends curl nginx
  ```
  
  ```text
  RUN apt-get update && apt-get install -y --no-install-recommends
  ```
  
  ```text
  RUN apt-get update && apt-get install -y --no-install-recommends \
      package-bar \
      package-baz \
      package-foo=1.3.*
  ```
  
  ```text
  RUN apt-get update && apt-get install -y --no-install-recommends \
      package-bar \
      package-baz \
      package-foo=1.3.*
  ```
  
  ```text
  RUN apt-get update && apt-get install -y --no-install-recommends \
      aufs-tools \
      automake \
      build-essential \
      curl \
      dpkg-sig \
      libcap-dev \
      libsqlite3-dev \
      mercurial \
      reprepro \
      ruby1.9.1 \
      ruby1.9.1-dev \
      s3cmd=1.1.* \
      && rm -rf /var/lib/apt/lists/*
  ```
  
  ```text
  RUN apt-get update && apt-get install -y --no-install-recommends \
      aufs-tools \
      automake \
      build-essential \
      curl \
      dpkg-sig \
      libcap-dev \
      libsqlite3-dev \
      mercurial \
      reprepro \
      ruby1.9.1 \
      ruby1.9.1-dev \
      s3cmd=1.1.* \
      && rm -rf /var/lib/apt/lists/*
  ```
  
  ```text
  RUN set -o pipefail && wget -O - https://some.site | wc -l > /number
  ```
  
  ```text
  RUN set -o pipefail && wget -O - https://some.site | wc -l > /number
  ```
  
  ```text
  RUN ["/bin/bash", "-c", "set -o pipefail && wget -O - https://some.site | wc -l > /number"]
  ```
  
  ```text
  RUN ["/bin/bash", "-c", "set -o pipefail && wget -O - https://some.site | wc -l > /number"]
  ```
  
  ```text
  ENV PG_MAJOR=9.3
  ENV PG_VERSION=9.3.4
  RUN curl -SL https://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src/postgres && …
  ENV PATH=/usr/local/postgres-$PG_MAJOR/bin:$PATH
  ```
  
  ```text
  ENV PG_MAJOR=9.3
  ENV PG_VERSION=9.3.4
  RUN curl -SL https://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src/postgres && …
  ENV PATH=/usr/local/postgres-$PG_MAJOR/bin:$PATH
  ```
  
  ```dockerfile
  # syntax=docker/dockerfile:1
  FROM alpine
  ENV ADMIN_USER="mark"
  RUN echo $ADMIN_USER > ./mark
  RUN unset ADMIN_USER
  ```
  
  ```dockerfile
  # syntax=docker/dockerfile:1
  FROM alpine
  ENV ADMIN_USER="mark"
  RUN echo $ADMIN_USER > ./mark
  RUN unset ADMIN_USER
  ```
  
  ```text
  $ docker run --rm test sh -c 'echo $ADMIN_USER'
  
  mark
  ```
  
  ```text
  $ docker run --rm test sh -c 'echo $ADMIN_USER'
  
  mark
  ```
  
  ```dockerfile
  # syntax=docker/dockerfile:1
  FROM alpine
  RUN export ADMIN_USER="mark" \
      && echo $ADMIN_USER > ./mark \
      && unset ADMIN_USER
  CMD sh
  ```
  
  ```dockerfile
  # syntax=docker/dockerfile:1
  FROM alpine
  RUN export ADMIN_USER="mark" \
      && echo $ADMIN_USER > ./mark \
      && unset ADMIN_USER
  CMD sh
  ```
  
  ```text
  RUN --mount=type=bind,source=requirements.txt,target=/tmp/requirements.txt \
      pip install --requirement /tmp/requirements.txt
  ```
  
  ```text
  RUN --mount=type=bind,source=requirements.txt,target=/tmp/requirements.txt \
      pip install --requirement /tmp/requirements.txt
  ```
  
  ```dockerfile
  # syntax=docker/dockerfile:1
  
  FROM scratch AS src
  ARG DOTNET_VERSION=8.0.0-preview.6.23329.7
  ADD --checksum=sha256:270d731bd08040c6a3228115de1f74b91cf441c584139ff8f8f6503447cebdbb \
      https://dotnetcli.azureedge.net/dotnet/Runtime/$DOTNET_VERSION/dotnet-runtime-$DOTNET_VERSION-linux-arm64.tar.gz /dotnet.tar.gz
  
  FROM mcr.microsoft.com/dotnet/runtime-deps:8.0.0-preview.6-bookworm-slim-arm64v8 AS installer
  
  # Retrieve .NET Runtime
  RUN --mount=from=src,target=/src <<EOF
  mkdir -p /dotnet
  tar -oxzf /src/dotnet.tar.gz -C /dotnet
  EOF
  
  FROM mcr.microsoft.com/dotnet/runtime-deps:8.0.0-preview.6-bookworm-slim-arm64v8
  
  COPY --from=installer /dotnet /usr/share/dotnet
  RUN ln -s /usr/share/dotnet/dotnet /usr/bin/dotnet
  ```
  
  ```dockerfile
  # syntax=docker/dockerfile:1
  
  FROM scratch AS src
  ARG DOTNET_VERSION=8.0.0-preview.6.23329.7
  ADD --checksum=sha256:270d731bd08040c6a3228115de1f74b91cf441c584139ff8f8f6503447cebdbb \
      https://dotnetcli.azureedge.net/dotnet/Runtime/$DOTNET_VERSION/dotnet-runtime-$DOTNET_VERSION-linux-arm64.tar.gz /dotnet.tar.gz
  
  FROM mcr.microsoft.com/dotnet/runtime-deps:8.0.0-preview.6-bookworm-slim-arm64v8 AS installer
  
  # Retrieve .NET Runtime
  RUN --mount=from=src,target=/src <<EOF
  mkdir -p /dotnet
  tar -oxzf /src/dotnet.tar.gz -C /dotnet
  EOF
  
  FROM mcr.microsoft.com/dotnet/runtime-deps:8.0.0-preview.6-bookworm-slim-arm64v8
  
  COPY --from=installer /dotnet /usr/share/dotnet
  RUN ln -s /usr/share/dotnet/dotnet /usr/bin/dotnet
  ```
  
  ```bash
  #!/bin/bash
  set -e
  
  if [ "$1" = 'postgres' ]; then
      chown -R postgres "$PGDATA"
  
      if [ -z "$(ls -A "$PGDATA")" ]; then
          gosu postgres initdb
      fi
  
      exec gosu postgres "$@"
  fi
  
  exec "$@"
  ```
  
  ```bash
  #!/bin/bash
  set -e
  
  if [ "$1" = 'postgres' ]; then
      chown -R postgres "$PGDATA"
  
      if [ -z "$(ls -A "$PGDATA")" ]; then
          gosu postgres initdb
      fi
  
      exec gosu postgres "$@"
  fi
  
  exec "$@"
  ```
  
  ```text
  COPY ./docker-entrypoint.sh /
  ENTRYPOINT ["/docker-entrypoint.sh"]
  CMD ["postgres"]
  ```
  
  ```text
  COPY ./docker-entrypoint.sh /
  ENTRYPOINT ["/docker-entrypoint.sh"]
  CMD ["postgres"]
  ```
  
  ```text
  RUN groupadd -r postgres && useradd --no-log-init -r -g postgres postgres
  ```
  
  ```text
  RUN groupadd -r postgres && useradd --no-log-init -r -g postgres postgres
  ```