title: apk python packages not installing in Alpine Docker Image
body: <p>I have a dockerfile as below:</p>
  
  <pre><code>FROM python:3.7.5-alpine3.10
  
  RUN apk update
  
  RUN mkdir -p /usr/src/app
  WORKDIR /usr/src/app
  
  RUN apk add --no-cache cython3
  
  CMD [ "sh", "ls"]
  </code></pre>
  
  <p>When I got into the container with <code>docker run -it --rm mycontainer /bin/sh</code> cython appears not to be installed. What am I missing?</p>
  
  <pre><code>/usr/src/app # which python
  /usr/local/bin/python
  /usr/src/app # python
  Python 3.7.5 (default, Oct 21 2019, 20:13:45) 
  [GCC 8.3.0] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  &gt;&gt;&gt; import cython
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
  ModuleNotFoundError: No module named 'cython'
  </code></pre>