Close the Docker Task Workflow Test's Five Undetected Shapes #3159

Context

tests/test_docker_task_workflow.py, which #3154 added for #3148, reads build-docker-task.yml's jobs and steps by indentation. The lane's third local review pass found five shapes the reader doesn't detect. #3154's body lists them, and they were left unfixed under the lane's fix-round budget.

Gaps

• The credential check joins its two empty-value tests with ||. Changing that to && isn't detected.
• The login step's username and password wiring isn't pinned, and neither is a non-Hub registry: value.
• Compact YAML with - name: at the same indent as steps: isn't parsed.
• Flow-style with: {push: true} isn't parsed.
• A run: step that pushes isn't counted as push-capable.

Possible Direction

Parse the workflow with a YAML loader where the test environment has one, rather than extending the indentation reader, and pin the login inputs and the check's operator. Keep each new assertion's mutation proof in-file, the way the existing cases do.