instruction: Create a Dockerfile for fundamentals
output: ```dockerfile
  from 172.17.0.5: icmp_seq=2 ttl=64 time=0.256 ms
  
  NoteIn the example, you had to install ping because it was not included
  in the container initially.Here, you used the ping command to ping the db container using its host entry,
  which resolves to 172.17.0.5. You can use this host entry to configure an application
  to make use of your db container.
  NoteYou can link multiple recipient containers to a single source. For
  example, you could have multiple (differently named) web containers attached to your
  db container.If you restart the source container, the /etc/hosts files on the linked containers
  are automatically updated with the source container's new IP address,
  allowing linked communication to continue.
  
  $ docker restart db
  db
  
  $ docker run -t -i --rm --link db:db training/webapp /bin/bash
  
  root@aed84ee21bde:/opt/webapp# cat /etc/hosts
  172.17.0.7 aed84ee21bde
  <...>
  172.17.0.9 db
  
  Request changesTable of contentsConnect using network port mappingConnect with the linking systemThe importance of namingCommunication across linksEnvironment variablesImportant notes on Docker environment variablesUpdating the /etc/hosts file
  
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