To conform with standard (and recently enforced) Python practices, Blubber's Python builder should always establish a virtual environment before performing its other operations.
The Debian bookworm container image wants people to do this (T346090) and at least one user has run into problems due to lack of virtualenv support (T346226).
docker-registry.discovery.wmnet/repos/releng/blubber/buildkit:v0.23.0 handles this.