Don't swallow stderr in gen_protos_docker

[dandavison]

This change allowed me to see the stderr (my docker storage was full).

---

Note

Stop capturing stderr during Docker image build in scripts/gen_protos_docker.py so error output is visible.

^{Written by Cursor Bugbot for commit ebf5499. This will update automatically on new commits. Configure here.}

[dandavison]

I don't have permissions to merge this btw.