feat: publish GitHub Action via Docker image

[maff]

Summary

• Move action.yml to repo root as a Docker action referencing ghcr.io/maff/git-that-semver:1
• Docker actions support dynamic outputs natively — all GTS_* keys are accessible to consumers without explicit declaration
• Add release workflow: validates semver tag (rejects build metadata), creates GitHub Release, updates floating v1 git tag for stable releases
• Slim Docker image from oven/bun:1-alpine to alpine:3 (~30MB vs ~150MB)
• CI git-that-semver job uses bun run index.ts directly (no bootstrapping risk)
• New test-action CI job builds Docker image locally and validates entrypoint for env, JSON, and YAML output modes
• Remove old .github/actions/git-that-semver/ nested action

Consumer usage

- uses: actions/checkout@v6
  with:
    fetch-depth: 0
- id: gts
  uses: maff/git-that-semver@v1
- run: echo "${{ steps.gts.outputs.GTS_DOCKER_VERSION }}"

First release steps

After merging:

1. git tag v1.0.0 && git push origin v1.0.0
2. build.yml pushes Docker image with tags 1.0.0, 1.0, 1, latest
3. release.yml creates GitHub Release and floating v1 git tag
4. uses: maff/git-that-semver@v1 is now available

Test plan

• bun test — all 153 tests pass
• bun run lint — formatting clean
• CI passes on this PR (test-action job validates Docker entrypoint)