title: exchangeable middle layer in docker
body: <p>Is it possible in Docker to do samething like this</p>
  
  <pre><code>Image1: Linux 
  Image2A: Python2.7 (FROM Image1)
  Image2B: Python3.6  (FROM Image1)
  Image3: install application  (FROM Image2A or FROM Image2B)
  
  Image4A: Linux&gt;Python2.7&gt;applciation (FROM Image3 with same parameters that depend python2.7)
  Image4B: Linux&gt;Python3.6&gt;applciation (FROM Image3 with same parameters that depend python3.6)
  </code></pre>
  
  <p>If is not possibile, how do recommend to plan it. Key point for me is that I have 
  exchangeable middle layers and want to have easy way to build last layer.</p>