chore(deps): update docker/build-push-action action to v6 (#5910)

Signed-off-by: milkmaker <milkmaker@mailcow.de>
Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

.github/workflows/rebuild_backup_image.yml

@@ -26,7 +26,7 @@ jobs:
           password: ${{ secrets.BACKUPIMAGEBUILD_ACTION_DOCKERHUB_TOKEN }}
 
       - name: Build and push
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           platforms: linux/amd64,linux/arm64