title: How do I link two running docker containers together?
body: <p>I'm wondering how to link docker containers that are already running.  Is this possible?  </p>
  
  <p>For example if I have 2 app (<code>app1</code> and <code>app2</code>) images and a single running <code>mongo</code> container, I can link them pretty easily when I <code>run</code> the them by doing the following:</p>
  
  <pre><code>docker run -d -name app1 --link mongo:mongo -p 8080:8080 app1
  docker run -d -name app2 --link mongo:mongo -p 8081:8081 app2
  </code></pre>
  
  <p>This works great.  However, suppose I have already ran <code>app1</code> and <code>app2</code> <em>without</em> linking them to the mongo container at the start, how do I go about linking the applications' containers to the running mongo container?</p>