title: ARG for folder path not working
body: <p>This works:</p>
  
  <pre><code>COPY Subfolder/ /inetpub/wwwroot
  </code></pre>
  
  <p>This copies the ENTIRE working directory (not just the Subfolder)</p>
  
  <pre><code>ARG site_root="Subfolder/"
  # ARG site_root="Subfolder/" # this doesnt work either
  # ARG site_root=Subfolder/ # this doesnt work either
  # ARG site_root=./Subfolder # this doesnt work either
  # ARG site_root=.Subfolder/ # this doesnt work either
  # ARG site_root=Subfolder # this doesnt work either
  COPY ${site_root} /inetpub/wwwroot
  </code></pre>
  
  <p>How do I use the subdir path in a var?</p>