title: Docker is not exporting env var
body: <p>I have built an image from this Dockerfile:</p>
  
  <pre><code>FROM jenkins
  
  RUN export http_proxy=http://user:pass@x.x.x.x:port
  
  #more things
  </code></pre>
  
  <p>But when I enter in the container (via exec /bin/bash), <code>http_proxy</code> is not exported.</p>
  
  <p>What can I do?</p>
  
  <p>Thanks</p>