[.github/actions/build-test-container]: Bump docker/build-push-action

Bumps the github-actions group in /.github/actions/build-test-container with 1 update: [docker/build-push-action](https://github.com/docker/build-push-action).


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

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

Signed-off-by: dependabot[bot] <[email protected]>

.github/actions/build-test-container/action.yaml

@@ -23,7 +23,7 @@ runs:
         path: /tmp/.buildx-cache
         key: ${{ runner.os }}-test-template-cache-${{ hashFiles('containers/test-template/**') }}-${{ inputs.python-version }}
     - name: Build Docker container for cache
-      uses: docker/build-push-action@v5
+      uses: docker/build-push-action@v6
       with:
         context: containers/test-template
         cache-from: type=local,src=/tmp/.buildx-cache