title: Pass ARG to ENTRYPOINT
body: <p>Say I have this in a Dockerfile:</p>
  
  <pre><code>ARG FOO=1
  ENTRYPOINT ["docker.r2g", "run"]
  </code></pre>
  
  <p>where I build the above with:</p>
  
  <pre><code>docker build -t "$tag" --build-arg FOO="$(date +%s)" .
  </code></pre>
  
  <p>is there a way to do something like:</p>
  
  <pre><code>ENTRYPOINT ["docker.r2g", "run", ARG FOO]  // something like this
  </code></pre>
  
  <p>I guess the argument could also be passed with <code>docker run</code> instead of during the <code>docker build</code> phase?</p>