Use release tags for Docker image publishing and add Docker skip markers

[Copilot]

This changes the Docker image publish path to use the auto-generated semantic release tag instead of the commit SHA. It also adds opt-out support so releases can skip image build/push when the merge commit includes #skip-docker or :noimage.

• Release-driven image publishing

  • Moves Docker image publishing behind the release workflow instead of publishing on every push to master.
  • Reuses the generated release tag as the image version tag for both images:
    • pg-with-backup:<release_tag>
    • pg-backup-walg:<release_tag>

• Reusable Docker publish workflow

  • Converts the Docker build/push workflow into a reusable workflow_call workflow.
  • Passes the release tag and source commit explicitly so image builds are tied to the merged commit that was tagged.

• Skip markers for image publishing

  • Adds commit-message based skip logic aligned with release marker patterns.
  • When the merge commit contains either #skip-docker or :noimage, the Docker build/push job is skipped while the GitHub release still proceeds.

• Release metadata

  • Adds Docker publish status to the generated release asset so it is clear whether images were published or intentionally skipped.

• Docs

  • Updates the build/publish documentation to reflect:
    • semantic version tags replacing SHA tags
    • release-triggered image publishing
    • #skip-docker / :noimage behavior

with:
  release_tag: v2.3.4
  source_ref: ${{ github.event.pull_request.merge_commit_sha }}
  skip_docker: ${{ needs.create-semver-tag.outputs.skip_docker == 'true' }}