title: How to set the python root to the Dockerfile workdir
body: <p>I have a simple Dockerfile:</p>
  
  <pre><code>FROM python:3.6-alpine
  RUN apk add -U --no-cache gcc build-base linux-headers ca-certificates python3-dev libffi-dev libressl-dev libxslt-dev
  COPY requirements.txt requirements.txt
  RUN pip install -r requirements.txt
  COPY . app/
  WORKDIR /app
  
  ENV FLASK_APP app.py
  
  EXPOSE 5000
  ENTRYPOINT [ "python" ]
  
  CMD ["-m", "flask", "run"]
  </code></pre>
  
  <p>When I run the app locally (not in docker) everything is fine. However, because the whole app is moved to the <code>/app</code> workdir, modules are not recognized anymore. I need to do this to make it work:</p>
  
  <pre><code>from app.my_module import MyClass
  </code></pre>
  
  <p>How do I tell python the root is <code>/app</code> and not <code>/</code>?</p>