Test the Docker Task Workflow's Credential Check, Login, and Hook Inputs #3148

Context

Raised by Copilot as a P2 finding on the develop -> main promotion #3146, against #3124, which gave build-docker-task.yml's docker-build-base job the Docker Hub credential check and login, and passed branch and platforms to the caller's hook. The finding asks for regression coverage of both. The gap is older than #3124: no test under tests/ names build-docker-task.yml on main or on develop, so the build-docker job's own credential check and login carry no test either.

Gap

A later edit could drop the login from either job, reorder it after the hook, or drop a hook input, and only a live run with a push would show it.

Possible Direction

A structural test in the shape tests/test_release_guards.py uses for other workflows: parse the workflow, and assert that each job pushing to Docker Hub runs the credential check and the login before any step that pushes, and that the hook step passes push, ref, branch, and platforms.