Here is a concept which you can follow. Suppose you make a Docker image for your Django app today, and end up using it for the following three weeks. Do you see your requirements.txt file being modified between now and then? Can you imagine a scenario in which you put out a hotpatch that comes with environmental changes?
As of Python 3.3, virtual-env is stdlib, which means it's very cheap to use, so I'd continue using it, just in case the Docker container isn't as disposable as you originally planned. Stated another way, even if your Docker-image pipeline is quite mature and the version of Python and dependencies are "pre-baked", it's such low-hanging fruit that while not explicitly necessary, it's worth sticking with best-practices.
I hope above explanation will help you resolve your query.