title: Docker: Run commands from multiple containers
body: <p>I want to execute a command that uses commands from multiple containers.</p>
  
  <p>E.g., I want to execute a backup script that used psql and pg_dump commands.</p>
  
  <pre><code>docker exec db_backup pg_dump
  failed to exec: exec: "pg_dump": executable file not found in $PATH
  </code></pre>
  
  <p><code>docker run</code> has an option <code>--link</code>. Is there a similar option for <code>exec</code>?</p>
  
  <p>To clear this up, there are 3 containers:</p>
  
  <ul>
  <li>my_app</li>
  <li>db</li>
  <li>db_backup</li>
  </ul>
  
  <p>I want to use pg commands located in <code>db</code> from my <code>db_backup</code> scripts.</p>