Once built a docker container is not expensive, but the idea with Docker is to create immutable, layered containers and rebuilding every time there is a change. Unlike with nodejs, it's harder to volume mount python packages installed in the docker container and so no one does it.

Change in requirements means rebuilding the container which is a very computationally and IO expensive process. So if you can get away with it virtualenv is better, but putting everything in a container so you can simply ship a container is also useful. It's a compromise.