title: Getting an error with docker: invalid reference format even though my command seems well-formatted
body: <p>I'm trying to run the following command and getting an error:</p>
  
  <pre><code>docker run --volumes-from wp-docker-tomthedev_wordpress_1 -v `pwd`/backup:/backup/ --rm ubuntu tar -cvf /backup/wp-docker-tomthedev.tar /var/www/
  </code></pre>
  
  <p>The error it's giving me is <code>docker: invalid reference format.</code></p>
  
  <p>From what I found by googling the error, I assume the mistake relies somewhere in either the syntax or the order of the flags but I tried with <code>--rm</code> right after <code>run</code> with no more success. </p>
  
  <p>Any help would be greatly appreciated :) </p>