title: Docker pull from privately hosted registry for python:3.6.8-slim based image works after push but returns blob unknown after prune
body: <p>For a cron job triggered script, I have used <code>python:3.6.8-slim</code> as a base image for my script to run.</p>
  
  <p>The script runs every hour and does so successfully until the <code>docker system prune</code> job runs.</p>
  
  <p>After that, the script fails to pull the image with the message <code>"ERROR: error pulling image configuration: unknown blob"</code></p>
  
  <p>When rebuilding and pushing the image to the registry again, the <code>docker pull</code> command runs without any problems until the prune job.</p>
  
  <p>I am using sonatype nexus3 as my private docker registry.</p>
  
  <p>I do not understand why the <code>docker system prune</code> job is causing this behaviour since the registry nexus3 is running in its very own container.</p>
  
  <p>my cron job:</p>
  
  <pre><code>30      *       *       *       *       docker pull my.registry.com/path/name:tag &amp;&amp; docker run --rm my.registry.com/path/name:tag
  </code></pre>
  
  <p>my <code>dockerfile</code>:</p>
  
  <pre><code>FROM python:3.6.8-slim
  WORKDIR /usr/src/app
  COPY requirements.txt ./
  RUN pip install --no-cache-dir -r requirements.txt
  COPY ./ ./src/
  CMD ["python", "src/myscript.py"]
  </code></pre>