title: docker exec is missing
body: <p>I have ubuntu 14.04 and it is missing <a href="https://docs.docker.com/reference/commandline/cli/#exec" rel="nofollow">docker exec</a></p>
  
  <pre><code>sudo docker exec -it ubuntu_bash bash
  </code></pre>
  
  <p>I wish to run interactive bash shell in existing running docker container.</p>
  
  <pre><code>sudo docker version
  Client version: 1.0.1
  Client API version: 1.12
  Go version (client): go1.2.1
  Git commit (client): 990021a
  Server version: 1.0.1
  Server API version: 1.12
  Go version (server): go1.2.1
  Git commit (server): 990021a
  </code></pre>