Summary

This is a rescue/follow-up for the stalled multi-arch workflow from #432, based on the direction from review feedback and the ImreSamu/docker-postgis-test prototype.

Changes:

• adds .github/workflows/test.yml while leaving the existing .github/workflows/main.yml untouched
• builds and tests release Docker images on ubuntu-24.04 and ubuntu-24.04-arm
• runs the docker-library/official-images tests before any publish-capable step
• keeps PR runs test-only; Docker Hub push/manifest/description updates are gated to canonical master non-PR runs
• publishes to postgis/docker-postgis-test when the gated path is enabled
• adds generated matrix.yml and teaches update.sh to regenerate it
• skips placeholder/development targets (17-3.6 Debian placeholder and *-master) from the required generated matrix
• uses run-id-scoped digest artifact names plus manifest concurrency to avoid cross-run collisions

Local validation

• bash -n update.sh ci/matrix.sh ci/test-image.sh ci/push-manifest.sh ci/prepare-dockerhub-readme.sh
• RUNNER_PLATFORMS_JSON='["ubuntu-24.04","ubuntu-24.04-arm"]' bash ci/matrix.sh
• Ruby YAML parse for .github/workflows/test.yml and matrix.yml
• git diff --check
• Local smoke build and official-images test for 17-3.5:
  • docker build -t postgis/postgis:ci-local-17-3.5-default 17-3.5
  • bash ci/test-image.sh postgis/postgis:ci-local-17-3.5-default
  • observed postgres-basics, postgres-initdb, and postgis-basics passing

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