build(deps): bump docker/metadata-action from 3 to 4

Bumps [docker/metadata-action](https://github.com/docker/metadata-action) from 3 to 4.
- [Release notes](https://github.com/docker/metadata-action/releases)
- [Upgrade guide](https://github.com/docker/metadata-action/blob/master/UPGRADE.md)
- [Commits](https://github.com/docker/metadata-action/compare/v3...v4)

---
updated-dependencies:
- dependency-name: docker/metadata-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/docker.yml

@@ -64,7 +64,7 @@ jobs:
         name: Docker meta
         if: github.event_name == 'push'
         id: meta
-        uses: docker/metadata-action@v3
+        uses: docker/metadata-action@v4
         with:
           images: etherpad/etherpad
           tags: |