title: docker: how to show the diffs between 2 images
body: <p>I have a Dockerfile with a sequence of RUN instructions that execute "apt-get install"s; for example, a couple of lines:</p>
  
  <pre><code>RUN apt-get install -y tree
  RUN apt-get install -y git
  </code></pre>
  
  <p>After having executed "docker build", if I then execute "docker images -a", I see the listing of all the base-child-child-.... images that were created during the build.</p>
  
  <p>I'd like to see a list of all of the packages that were installed when the "apt-get install -y git" line was executed (including the dependent packages that may have also been installed, besides the git packages).  </p>
  
  <p>Note: I believe that the "docker diff" command shows the diffs between a container and the image from which it was started. Instead I'd like the diffs between 2 images (of the same lineage): the "tree" and "git" image IDs. Is this possible?</p>
  
  <p>Thanks.</p>