title: Install module from sibling directory
body: <p>I would like to have a following structure in a project:</p>
  <pre><code>/
      lib/
          common/
              __init__.py
              common.py
          setup.py
      alpha/
          main.py
          Dockerfile
      beta/
          main.py
          Dockerfile
      Jenkinsfile
  </code></pre>
  <p>In the build process I want to build a module <code>common</code> and then use that as a dependency of <code>alpha</code> and <code>beta</code>.</p>
  <p>I don't want to have <code>common</code> as a separate project/repo. In fact, this is my current state and I'm using <code>pip install git+https://&lt;repo&gt;</code> - but I decided I prefer components to share lifecycle, so to keep them together. And I think pip install from Git is not too the cleanest approach either.</p>
  <p>The simplest thing might be to copy entire lib into alpha/beta and just use the source. Not being expert in Python I can't say how &quot;nice&quot; this would be, but anyway running <code>docker build</code> from within subdirectory doesn't give me access to any context outside of that subdirectory.</p>
  <p>Can you suggest an efficient and clean approach?</p>