Bump docker/build-push-action from 6.8.0 to 6.9.0 (#59)

Bumps [docker/build-push-action](https://github.com/docker/build-push-action) from 6.8.0 to 6.9.0.
- [Release notes](https://github.com/docker/build-push-action/releases)
- [Commits](docker/build-push-action@32945a3...4f58ea7)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Jakub Hrozek <jakub@stacklok.com>

Author: dependabot[bot]

.github/workflows/testbuild.yml

@@ -40,7 +40,7 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@c47758b77c9736f4b2ef4073d4d51994fabfe349 # v3
       - name: Build Docker image
-        uses: docker/build-push-action@32945a339266b759abcbdc89316275140b0fc960 # v6
+        uses: docker/build-push-action@4f58ea79222b3b9dc2c8bbdd6debcef730109a75 # v6
         with:
           context: .
           push: false