title: Containers are not linked with docker-compose version 2
body: <p>I have a docker-compose file that I upgraded from version 1 to version 2.</p>
  
  <p>It set ups a simple Selenium hub with a firefox node.</p>
  
  <p>When I set it up as version 1 it launches fine. When I set it up with version 2 the ff container returns <code>"Not linked with a running Hub container"</code> and exits.</p>
  
  <p>As I researched it and understood it , is that the linkage between the containers somehow suffers.</p>
  
  <p>Is there a solution ?? Am I missing something ??</p>
  
  <pre><code>version: '2'
  services:
    hub:
      container_name: hub
      image: selenium/hub 
      ports:
        - "8080:4444" # HOST:CONTAINER
      expose:
        - "4444" 
  
    ff:
      container_name: ff
      image: selenium/node-firefox 
      links:
        - hub
      expose:
        - "5555" 
  </code></pre>