It seems like there is no way to define that a Task requires docker support from an agent, right?

Correct, basically the idea is you either have workers working in venv mode or docker.
If you have a mixture of the two, then you can have the venv agents pulling from one queue (say default_venv) and the docker mode agents pulling from a different queue (say default_docker). This way you always know what you are getting when you enqueue your Task