ci(rds): PR dry-run + workflow_dispatch test path for postgres image publish

[vieiralucas]

Follow-up to #803. Verifies the prebuilt-postgres-image publish path before we cut a release.

Summary

• New pull_request trigger on .github/workflows/docker-rds-images.yml (paths-filtered to the workflow file + crates/fakecloud-rds/assets/postgres/**). Runs the full multi-arch build for all 4 postgres majors with push: false — catches Dockerfile typos and workflow syntax regressions before a release tag fires.
• workflow_dispatch now publishes to ghcr.io but tagged as <major>-dev-<short-sha> instead of <major>-<version>. Also skips the rolling <major> tag. End-to-end validation of the push + manifest-merge path without polluting release tags.
• On a real v* tag push, behaviour is unchanged: <major>-<version> pinned + rolling <major>.

Test plan

• CI dry-run (this PR) builds all 8 (4 majors × 2 arches) jobs successfully → confirms the multi-arch build path works.
• After merge: trigger docker-rds-images.yml via workflow_dispatch against main. Expect it to publish 4 images tagged <major>-dev-<short-sha> to ghcr.io/faiscadev/fakecloud-postgres.
• Manually docker pull ghcr.io/faiscadev/fakecloud-postgres:16-dev-<sha> and confirm both arches are present in the manifest list.
• If both pass → cut release v0.13.1. The real release will exercise the same publish path with the proper <major>-<version> + rolling <major> tags.

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Summary by cubic

Adds a PR dry-run and a dev-tagged workflow_dispatch path to validate the prebuilt Postgres image pipeline before releases. Real release tags still publish <major>-<version> plus rolling <major> with no behavior change.

• New Features
  • PRs touching the workflow or crates/fakecloud-rds/assets/postgres/** run the full multi-arch build for Postgres 13–16 on linux/amd64 and linux/arm64 with no push, to catch build/workflow issues early.
  • workflow_dispatch publishes to ghcr.io/faiscadev/fakecloud-postgres with <major>-dev-<short-sha> tags and merged multi-arch manifests, skipping the rolling <major> tag.

^{Written for commit 867ce12. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 1 file

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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