title: add-apt-repository fails during docker build
body: <p>I am trying to get the <a href="https://hub.docker.com/r/dordoka/tomcat/" rel="nofollow noreferrer">dordoka/tomcat</a> docker image up and running, but  I get this error when running <code>docker build</code>:</p>
  
  <pre><code>Cannot add PPA: 'ppa:~webupd8team/ubuntu/y-ppa-manager'.
  ERROR: '~webupd8team' user or team does not exist.
  The command '/bin/sh -c apt-get update &amp;&amp; 
  apt-get install -y software-properties-common &amp;&amp;
  add-apt-repository -y ppa:webupd8team/y-ppa-manager &amp;&amp;
  add-apt-repository -y ppa:webupd8team/ubuntu/y-ppa-manager &amp;&amp;
  apt-get update &amp;&amp;
  apt-get install -y git build-essential curl wget software-properties-common'
  returned a non-zero code: 1
  </code></pre>
  
  <p>The command that is failing is <code>add-apt-repository -y ppa:webupd8team/ubuntu/y-ppa-manager</code>. This command runs fine outside of docker. The proxy is configured correctly, as far as I can tell. Any ideas?</p>