Stop pinning exact digest in helper image config test

Lachlan Grantrequested to merge
39590-fix-stale-helper-image-digest-test into main

Summary

TestDockerCommandWithHelperImageConfig asserted an exact image ID and manifest digest pair for the mutable Docker Hub tag gitlab/gitlab-runner-helper:x86_64-v16.9.1. That tag was republished upstream (the manifest digest and image ID are now the same value, where they used to differ), which permanently breaks the hardcoded assertion on any pipeline running Docker executor integration tests, regardless of the change under test. Observed failing on an unrelated MR: https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/15212124101

Fix

Replace the exact assert.Contains match with a regex (assert.Regexp, already used elsewhere in this file) that verifies the same intent — the configured helper image tag was pulled and used — without pinning specific hash values outside this repository's control.

Verification

  • go build / go vet clean, golangci-lint 0 issues.
  • Verified the regex matches both the old log format (be0a1939... / 24432bb8...) and the new one (24432bb8... / 24432bb8...), and correctly rejects a mismatched tag.

Closes #39590 (closed)

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