title: Docker echo environment variable
body: <p>I'm trying to write a little docker file that sets a User and just echos the current user as a little example to prove to myself it is working. I've tried a number of variants and couldn't find much help in the documentation.</p>
  
  <pre><code>FROM ubuntu
  USER daemon
  # ENTRYPOINT ["echo", "$USER"]
  # just gives "$USER"
  # ENTRYPOINT ["echo", "-e", "${USER}"]
  # just gives "$USER"
  # ENTRYPOINT echo $USER
  # gives empty string
  # ENTRYPOINT ["/bin/echo", "$USER"]
  # just gives "$USER"
  </code></pre>
  
  <p>I'm running <code>docker build .</code> on the dockerfile and then running <code>docker run &lt;image-id&gt;</code> and getting the results </p>
  
  <p>Expected result is <code>daemon</code>, or without the <code>USER daemon</code> line, I expect <code>root</code>. Probably a really simple answer.</p>