title: Big size of python image in Docker
body: <p>I want to test my app with <strong>Docker</strong>. So, I have this in <strong>Dockerfile</strong>:</p>
  
  <pre><code>FROM python:3-onbuild
  CMD [ "python", "./test.py" ]
  </code></pre>
  
  <p><strong>test.py:</strong></p>
  
  <pre><code>print(123)
  </code></pre>
  
  <p>Then I run:</p>
  
  <pre><code>docker build -t my_test_app .
  </code></pre>
  
  <p>So, I have one big image. <code>docker images</code> return:</p>
  
  <pre><code>REPOSITORY          TAG                 IMAGE ID        CREATED    VIRTUAL SIZE
  python              3-onbuild           b258eb0a5195    8 days ago 757 MB
  </code></pre>
  
  <p>Why is the file size so large? </p>
  
  <p>Is that file size normal?</p>