title: Docker add fails with no such file or directory
body: <p>I have a docker file that I have used many many times to build an image. But now when I try and build it fails with:</p>
  
  <pre><code>Step 18/25 : ADD mysql /var/lib/mysql
  ADD failed: stat /var/lib/docker/tmp/docker-builder258868311/mysql: no such file or directory
  </code></pre>
  
  <p>The mysql dir does exist locally and it is readable. What could have changed that is now causing this? How can I debug the problem? </p>