title: invalid reference format when building a Docker Image
body: <p>I have the following lines in a Dockerfile:</p>
  
  <pre><code>ARG SOMEARGUMENT
  COPY --from=$SOMEARGUMENT /some/path/goes/here
  </code></pre>
  
  <p>where SOMEARGUMENT could be either "some/repository:1.1.1" or "some/repository:2.2.2"</p>
  
  <p>But when trying to build I always get the error:</p>
  
  <pre><code>invalid reference format: repository name must be lowercase
  </code></pre>
  
  <p>So it is obvious that the argument passed when building the image (i.e. SOMEARGUMENT) is not being resolved.</p>
  
  <p>Is it actaully possible to do something like this in a Dockerfile?</p>
  
  <p>Thanks.</p>