instruction: Create a comprehensive Dockerfile for networking applications with best practices
output: ```dockerfile
  from using different image versions.If you don't specify a version at all, by convention the image's latest tag
  is resolved to a digest. Workers use the image at this digest when creating
  the service task.Thus, the following two commands are equivalent:
  
  $ docker service create --name="myservice" ubuntu
  
  $ docker service create --name="myservice" ubuntu:latest
  If you specify a digest directly, that exact version of the image is always
  used when creating service tasks.
  
  $ docker service create \
   --name="myservice" \
   ubuntu:16.04@sha256:35bc48a1ca97c3971611dc4662d08d131869daa692acb281c7e9e052924e38b1
  When you create a service, the image's tag is resolved to the specific digest
  the tag points to at the time of service creation. Worker nodes for that
  service use that specific digest forever unless the service is explicitly
  updated. This feature is particularly important if you do use often-changing tags
  such as latest, because it ensures that all service tasks use the same version
  of the image.
  NoteIf content trust is
  enabled, the client actually resolves the image's tag to a digest before
  contacting the swarm manager, to verify that the image is signed.
  Thus, if you use content trust, the swarm manager receives the request
  pre-resolved. In this case, if the client cannot resolve the image to a
  digest, the request fails.If the manager can't resolve the tag to a digest, each worker
  node is responsible for resolving the tag to a digest, and different nodes may
  use different versions of the image. If this happens, a warning like the
  following is logged, substituting the placeholders for real information.
  
  unable to pin image <IMAGE-NAME> to digest: <REASON>To see an image's current digest, issue the command
  docker inspect <IMAGE>:<TAG> and look for the RepoDigests line. The
  following is the current digest for ubuntu:latest at the time this content
  was written. The output is truncated for clarity.
  
  $ docker inspect ubuntu:latest
  
  "RepoDigests": [
   "ubuntu@sha256:35bc48a1ca97c3971611dc4662d08d131869daa692acb281c7e9e052924e38b1"
  ],After you create a service, its image is never updated unless you explicitly run
  docker service update with the --image flag as described below. Other update
  operations such as scaling the service, adding or removing networks or volumes,
  renaming the service, or any other type of update operation do not update the
  service's image.Update a service's image after creationEach tag represents a digest, similar to a Git hash. Some tags, such as
  latest, are updated often to point to a new digest. Others, such as
  ubuntu:16.04, represent a released software version and are not expected to
  update to point to a new digest often if at all. When you create a service, it
  is constrained to create tasks using a specific digest of an image until you
  update the service using service update with the --image flag.When you run service update with the --image flag, the swarm manager queries
  Docker Hub or your private Docker registry for the digest the tag currently
  points to and updates the service tasks to use that digest.
  NoteIf you use content trust, the Docker
  client resolves image and the swarm manager receives the image and digest,
  rather than a tag.Usually, the manager can resolve the tag to a new digest and the service
  updates, redeploying each task to use the new image. If the manager can't
  resolve the tag or some other problem occurs, the next two sections outline what
  to expect.If the manager resolves the tagIf the swarm manager can resolve the image tag to a digest, it instructs the
  worker nodes to redeploy the tasks and use the image at that digest.If a worker has cached the image at that digest, it uses it.If not, it attempts to pull the image
  ```