The issue is from the CMD you are using on your Dockerfile. If I explain it badly, CMD doesn’t run the command you’re giving in a shell, and thus, doesn’t interpret the $PORT variable.
For that, you need /bin/bash (or any other shell that match your preference) to execute the command gunicorn run:app –bind 0.0.0.0:$PORT. There are several solutions for that:
Solution 1
The one I recommend the most.
Create a new file in your repository that will be responsible for launching your application. For the sake of the demonstration, I will call it scripts/docker-cmd
The content of this file should be:
#!/bin/bash
gunicorn run:app –bind 0.0.0.0:$PORT
Be mindful that this file must be executable. Otherwise, it will fail!
Then, change the CMD operation in your Dockerfile to this:
CMD ["/app/scripts/docker-cmd"]
Solution 2
Change the CMD operation to this:
CMD ["/bin/bash", "-c", "gunicorn run:app –bind 0.0.0.0:$PORT"]
Solution 3
Easier, but it could break in the future if you’re not careful.
Replace the CMD operation by the following block:
ENTRYPOINT ["/bin/bash", "-c"]
CMD ["gunicorn run:app –bind 0.0.0.0:$PORT"]
Hope that helps,