You're correct, theoretically. But in reality is best to not stick a dozen services or apps in a single docker image.
It’s not really about services as it is about things that you might need *for the service*. For instance, maybe you want node installed so you can build some static files using node.js tooling, but node requires gyp which requires Python. Now you install a new version of setuptools that breaks the OS installed gyp and suddenly now you can’t build your static files anymore.
A virtual environment costs very little and protects you from these things. Of course, if you’re not using the System python (for instance, the official Python images install their own Python into /usr/local) then there’s no reason to use a virtual environment because you’re already isolated from the system.