ci: fix action tests for tag push events

[Copilot]

Action tests were failing on tag pushes (e.g., v1.0.0) because assertions hardcoded "main" as the expected tag for non-PR events.

Changes

• Test assertions: Use github.ref_name dynamically and detect tags via github.ref.startsWith('refs/tags/')
  • __test-action-docker-build-image.yml
  • __test-action-get-image-metadata.yml
• Documentation: Added CI testing strategy section to README.md explaining arrange-act-assert pattern and tag behavior
• AGENTS.md: Reference new CI testing docs

Logic

// Before: hardcoded "main"
expectedTags.push("main");

// After: dynamic with tag detection
const refTag = `${{ github.ref_name }}`;
expectedTags.push(refTag);

const isTag = `${{ github.ref }}`.startsWith('refs/tags/');
const isPushOnDefaultBranch = `${{ github.event_name }}` === "push" && !isTag && refTag === "${{ github.event.repository.default_branch }}";

if (isPushOnDefaultBranch) {
  expectedTags.push("latest");
}

Result: Tests now expect correct tags for all contexts:

• Branch main → ["main", "latest"]
• Tag v1.0.0 → ["v1.0.0"]
• PR chore(deps): bump docker/setup-qemu-action from 2 to 3 in /actions/docker/create-images-manifests #123 → ["pr-123-{sha}", "pr-123"]

Original prompt

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This section details on the original issue you should resolve

<issue_title>ci: fix action tests when running on tag pushes</issue_title>
<issue_description>CI jobs for testing our local GitHub Actions are currently failing whenever they run in a tag push context. The arrange/act/assert tests that cover our actions use assumptions tailored for branch pushes, so the assertions break on tag pushes.

Recent failing runs:

• https://github.com/hoverkraft-tech/ci-github-container/actions/runs/19180597951/job/54836430569
• https://github.com/hoverkraft-tech/ci-github-container/actions/runs/19180597951/job/54836286047
• https://github.com/hoverkraft-tech/ci-github-container/actions/runs/19180597951/job/54836437498

Tasks:

• Update README.md to document how CI tests our local GitHub Actions: workflows are testing (arrange act assert) some local github actions.
• Update AGENTS.md to reference the new README details.
• Analyse the failing jobs and adjust the assertions so tests handle tag pushes correctly.
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ci: fix action tests when running on tag pushes #567

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