title: what's the double arrow (<<: *django) means in dockerfile?
body: <p>I see something like </p>
  
  <pre><code>   celerybeat:
       &lt;&lt;: *django
  </code></pre>
  
  <p>in <a href="https://github.com/pydanny/cookiecutter-django" rel="noreferrer">https://github.com/pydanny/cookiecutter-django</a> example docker files.</p>
  
  <p>What does it mean? I can't google <code>&lt;&lt;: *</code> </p>