title: Python docker image doesn't find opencv when installed with apt-get
body: <p>My Dockerfile:</p>
  
  <pre><code>FROM python:3.7
  
  RUN apt-get update &amp;&amp; apt-get install -y python3-opencv
  </code></pre>
  
  <p>Build this then get the following result:</p>
  
  <pre class="lang-sh prettyprint-override"><code>$ docker run -it myimage:opencv python3 -c "import cv2"
  Traceback (most recent call last):
    File "&lt;string&gt;", line 1, in &lt;module&gt;
  ModuleNotFoundError: No module named 'cv2'
  
  $ docker run -it hazopticnano:opencv python3 -c "import sys; print(sys.path)"
  ['', '/usr/local/lib/python37.zip', '/usr/local/lib/python3.7', '/usr/local/lib/python3.7/lib-dynload', '/usr/local/lib/python3.7/site-packages']
  </code></pre>
  
  <p>Within the container:</p>
  
  <pre class="lang-sh prettyprint-override"><code>$ ls usr/lib/python3/dist-packages/
  cv2.cpython-37m-x86_64-linux-gnu.so  numpy  numpy-1.16.2.egg-info  pkg_resources
  </code></pre>
  
  <p>If I run this then it will work:</p>
  
  <pre class="lang-sh prettyprint-override"><code>$ ln -s /usr/lib/python3/dist-packages/* /usr/local/lib/python3.7/dist-packages/
  </code></pre>
  
  <p>Is there a way I can make this work "out of the box", without having to run the last command?</p>