make it easier to update image_test hashes

[zevdg]

We had to do this recently with #2098. This is also useful for organisations that link against different zlib implementations (namely zlib-ng). This change makes it possible to easily swap out the hashes without modifying the test code.

PR Checklist

Please check if your PR fulfills the following requirements:

• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [x] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [ ] Documentation content changes
• [ ] Other... Please describe:

What is the current behavior?

test hashes are hardcoded into test, and must be updated manually

Issue Number: N/A

What is the new behavior?

test hashes are now encoded in testdata/goldens and can be regenerated by the test when needed.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

[zevdg]

Ok, this is ready for reviewer now.

[github-actions[bot]]

This Pull Request has been automatically marked as stale because it has not had any activity for 180 days. It will be closed if no further activity occurs in 30 days. Collaborators can add an assignee to keep this open indefinitely. Thanks for your contributions to rules_docker!

[github-actions[bot]]

This PR was automatically closed because it went 30 days without a reply since it was labeled "Can Close?"