title: run apt-get with proxy in Dockerfile
body: <p>I am behind a proxy an i need to install something via <code>apt-get</code>.</p>
  
  <p>The best I came with is this</p>
  
  <pre><code>ARG PROXY
  ENV http_proxy=$PROXY
  ENV https_proxy=$PROXY
  RUN apt-get update -y &amp;&amp; apt-get -y install ...
  ENV http_proxy=
  ENV https_proxy=
  </code></pre>
  
  <p>The thing is that I need to unset those environment variables afterwards.</p>
  
  <p>Any idea how to do it in less then 5 layers?</p>