Unable to run pytests in a docker container

I built a docker image and tried to run pytest, I am getting the following error :
docker run docker-whale

============================= 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

My dockerFile:

CMD ["pytest" , "--env preview" , "fe/tests/US/MyCom/MyPageTests/" , "-m preview"]

Is this happening due to conflict between docker “env” variable and my pytest “env” environment variable? If so, what is the work-around?

The same command works fine if I ssh into the docker container and run it.

Source: StackOverflow