title: tar: Cannot read: Is a directory in docker
body: <p>I want to add an archive (<code>.tar.gz</code>) and unpack it during image creation using a Dockerfile.</p>
  
  <pre><code>ADD archive.tar.gz /archive.tar.gz
  RUN tar xzf archive.tar.gz
  </code></pre>
  
  <p>When I want to unpack the archive, I get following message:</p>
  
  <pre><code>tar: /archive.tar.gz: Cannot read: Is a directory
  tar: At beginning of tape, quitting now
  tar: Error is not recoverable: exiting now
  </code></pre>
  
  <p>which doesn't make any sense to me, since the file <code>archive.tar.gz</code> is not a directory.</p>