Verify resources: authenticate Docker Hub image checks

What does this MR do?

Authenticates the Docker Hub image checks in mage resources:verifyAll (magefiles/build/checker.go), which are currently done unauthenticated and hit Docker Hub'''s anonymous pull rate limit.

Why was this MR needed?

verify stable resources / verify bleeding edge resources check roughly 160 Docker Hub tags per run via unauthenticated skopeo inspect calls. Docker Hub'''s anonymous limit is 100 pulls/6h per IP, so this job exceeds the limit on its own, regardless of any other traffic sharing the runner'''s IP - see the linked issue for a real failing job.

This attaches DOCKER_HUB_USER/DOCKER_HUB_PASSWORD (already used to push these images in .gitlab/ci/release.gitlab-ci.yml) the same way CI_REGISTRY_USER/CI_REGISTRY_PASSWORD are already attached for registry.gitlab.com images, keeping both credential sets mutually exclusive per registry (see MR history note below).

What are the relevant issue numbers?

#39607 (closed)

Notes for reviewers

This credential-attach logic has caused one prior regression: !5906 (merged) attached GitLab.com credentials to every image check unconditionally, which broke Docker Hub checks with 401s; fixed by !5914 (merged) by scoping credentials to registry.gitlab.com only. This MR adds a second, equally scoped switch case for Docker Hub, and a new checker_test.go with table-driven tests asserting credentials never cross registries, to guard against a repeat of that failure mode.

Still needs verification before merge: confirm DOCKER_HUB_USER/ DOCKER_HUB_PASSWORD are actually exposed to the verify bleeding edge resources/verify stable resources jobs (their protection/environment scope isn'''t visible from the repo). Marked draft until that'''s confirmed against a real pipeline run.

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