title: gunicorn not found when running a docker container with venv
body: <p>I'm trying to run a Docker container which complains with the error message: <code>/bin/sh: gunicorn: not found</code>. Firing up the server locally without Docker works fine. And building the image also works fine. I'm new to docker so I don't know if anything looks weird in my Dockerfile..</p>
  
  <p>My Dockerfile:</p>
  
  <pre><code>FROM python:3.7-alpine
  
  RUN adduser -D teamreacher
  WORKDIR /home/teamreacher
  
  # copy and install dependencies
  COPY ./requirements.txt requirements.txt
  RUN python -m venv venv
  RUN venv/bin/pip install --upgrade pip
  RUN venv/bin/pip install -r requirements.txt
  
  # copy the app
  COPY . .
  RUN chmod +x boot.sh
  
  RUN chown -R teamreacher:teamreacher ./
  USER teamreacher
  
  # expose port and run server
  EXPOSE 5000
  
  RUN source venv/bin/activate
  CMD gunicorn -b :5000 --access-logfile - --error-logfile - wsgi:app
  </code></pre>
  
  <p>And my requirements.txt:</p>
  
  <pre><code>Flask==1.0.2
  Flask-RESTful==0.3.6
  Flask-SQLAlchemy==2.3.2
  Flask-JWT==0.3.2
  Flask-Cors==3.0.7
  gunicorn==19.9.0
  </code></pre>