title: docker run python script in CMD with environment variables
body: <p>I'm trying to run a simple Dockerfile that runs a python script and takes some environment variable as an argument (using argparser):</p>
  
  <pre><code>FROM python:2.7
  COPY . /app
  WORKDIR /app
  RUN pip install argparse
  ENV POOL "pool_argument"
  CMD ["python", "script.py", "--pool", "${POOL}"]
  </code></pre>
  
  <p>and my python script <code>script.py</code>:</p>
  
  <pre><code>import argparse
  
  parser = argparse.ArgumentParser()
  parser.add_argument('--pool', required=True)
  known_args, unknown_args = parser.parse_known_args()
  print("Args: {}".format(known_args))
  </code></pre>
  
  <p>after building and running, I'm getting:</p>
  
  <pre><code>Args: Namespace(pool='${POOL}')
  </code></pre>
  
  <p>I tried many variations of this, but none seemed to work.</p>