title: Removing what was added in previous layer in docker
body: <p>I'm trying to reduce the size of my docker image.  In my docker file, I do this:</p>
  <pre><code>FROM crystal/centos
  MAINTAINER crystal
  
  ADD ./rpms/test.rpm ./rpms/ 
  RUN yum -y --nogpgcheck localinstall /rpms/test.rpm 
  </code></pre>
  <p>from what I understand, the ADD command is in its own layer, and then RUN is in another layer.  So after I install the rpm, how do I go about deleting the initial <code>/rpms</code> directory.</p>