build(deps): bump docker/build-push-action from 5 to 6

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 5 to 6.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Commits](https://github.com/docker/build-push-action/compare/v5...v6)

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

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

.github/workflows/docker.yml

@@ -38,7 +38,7 @@ jobs:
 
       # https://github.com/docker/build-push-action
       - name: Build Docker image
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           tags: jupyter/repo2docker:pr

.github/workflows/release.yml

@@ -124,7 +124,7 @@ jobs:
           echo "TAGS=$TAGS" >> $GITHUB_ENV
 
       - name: Build and push repo2docker
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           platforms: linux/amd64,linux/arm64