title: Copy a file from container to host during build process
body: <p>How do I copy files from a docker container to the host machine during <code>docker build</code> command? </p>
  
  <p>As a part of building my docker image for my app, I'm running some tests inside it, and I would like to copy the output of the test run into the host (which is a continuous integration server) to do some reporting. </p>