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/build.yml

@@ -132,7 +132,7 @@ jobs:
           if [[ "$GITHUB_REF" == refs/heads/master ]]; then
             echo "::set-output name=latest::type=raw,latest"
           fi
-      - uses: docker/metadata-action@v3
+      - uses: docker/metadata-action@v4
         id: meta
         with:
           images: zncbouncer/znc-git