title: Docker ERROR: unsatisfiable constraints when using apk
body: <p>I am using this as my Dockerfile. On building it, the code exists at step 2.</p>
  
  <pre><code>FROM python:3.7-alpine
  RUN apk update &amp;&amp; apk add --no-cache gcc libffi-dev g++ python-dev build-base linux-headers postgresql-dev postgresql-contrib pcre-dev bash alpine-sdk jpeg-dev zlib-dev \ &amp;&amp; pip install wheel
  
  COPY ./requirements.txt .
  
  RUN pip install -r requirements.txt
  </code></pre>
  
  <p>This is what I get on the terminal</p>
  
  <pre><code>Building web
  Step 1/18 : FROM python:3.7-alpine
   ---&gt; 7fbc871584eb
  Step 2/18 : RUN apk update &amp;&amp; apk add --no-cache gcc libressl libffi-dev g++ python-dev build-base linux-headers postgresql-dev postgresql-contrib pcre-dev bash alpine-sdk jpeg-dev zlib-dev \ &amp;&amp; pip install wheel
   ---&gt; Running in 102a327c46d9
  fetch http://dl-cdn.alpinelinux.org/alpine/v3.11/main/x86_64/APKINDEX.tar.gz
  fetch http://dl-cdn.alpinelinux.org/alpine/v3.11/community/x86_64/APKINDEX.tar.gz
  v3.11.5-56-gd94bd50429 [http://dl-cdn.alpinelinux.org/alpine/v3.11/main]
  v3.11.5-54-g70c5111c08 [http://dl-cdn.alpinelinux.org/alpine/v3.11/community]
  OK: 11274 distinct packages available
  fetch http://dl-cdn.alpinelinux.org/alpine/v3.11/main/x86_64/APKINDEX.tar.gz
  fetch http://dl-cdn.alpinelinux.org/alpine/v3.11/community/x86_64/APKINDEX.tar.gz
  ERROR: unsatisfiable constraints:
     (missing):
      required by: world[]
  ERROR: Service 'web' failed to build: The command '/bin/sh -c apk update &amp;&amp; apk add --no-cache gcc libressl libffi-dev g++ python-dev build-base linux-headers postgresql-dev postgresql-contrib pcre-dev bash alpine-sdk jpeg-dev zlib-dev \ &amp;&amp; pip install wheel' returned a non-zero code: 1
  </code></pre>
  
  <p>My Docker version 19.03.3</p>