instruction: Create a Dockerfile for fundamentals
output: ```dockerfile
  from the alias you specified above. If the alias
  were db1, the variables would be prefixed with DB1_. You can use these
  environment variables to configure your applications to connect to the database
  on the db container. The connection is secure and private; only the
  linked web container can communicate with the db container.Important notes on Docker environment variablesUnlike host entries in the /etc/hosts file,
  IP addresses stored in the environment variables are not automatically updated
  if the source container is restarted. We recommend using the host entries in
  /etc/hosts to resolve the IP address of linked containers.These environment variables are only set for the first process in the
  container. Some daemons, such as sshd, scrub them when spawning shells
  for connection.Updating the /etc/hosts fileIn addition to the environment variables, Docker adds a host entry for the
  source container to the /etc/hosts file. Here's an entry for the web
  container:
  
  $ docker run -t -i --rm --link db:webdb training/webapp /bin/bash
  
  root@aed84ee21bde:/opt/webapp# cat /etc/hosts
  172.17.0.7 aed84ee21bde
  <...>
  172.17.0.5 webdb 6e5cdeb2d300 db
  You can see two relevant host entries. The first is an entry for the web
  container that uses the Container ID as a host name. The second entry uses the
  link alias to reference the IP address of the db container. In addition to
  the alias you provide, the linked container's name, if unique
  ```