Verify resources: authenticate Docker Hub image checks

Daniel Keenanrequested to merge
cherry-pick-9713b554 into 19-2-stable

What does this MR do?

Cherry-picks !6967 (merged) into 19-2-stable.

Why was this MR needed?

To fix the currently failing verify stable resources job in future patch release pipelines derived from 19-2-stable.

See thread for more context: https://gitlab.com/gitlab-com/runner-group/team-tasks/-/work_items/490#note_3629516017

What's the best way to test this MR?

N/A

What are the relevant issue numbers?

N/A

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