title: Unable to run pytests in a docker container
body: <p>I built a docker image and tried to run pytest, I am getting the following error :
  docker run docker-whale</p>
  
  <blockquote>
    <p>============================= test session starts ==============================
    platform linux2 -- Python 2.7.16, pytest-4.4.0, py-1.8.0, pluggy-0.9.0
    rootdir: /app
    ========================= no tests ran in 0.00 seconds =========================
    ERROR: file not found: --env preview</p>
  </blockquote>
  
  <p>My dockerFile:</p>
  
  <pre><code>CMD ["pytest" , "--env preview" , "fe/tests/US/MyCom/MyPageTests/" , "-m preview"]
  </code></pre>
  
  <p>Is this happening due to conflict between docker "env" variable and my pytest "env" environment variable? If so, what is the work-around?</p>
  
  <p>The same command works fine if I ssh into the docker container and run it.</p>