Update docker/build-push-action action to v6 (#63)

skipping CI since this doesn't include any changes to our images.

[skip ci]

Author: renovate[bot]

.github/workflows/build.yaml

@@ -114,7 +114,7 @@ jobs:
       - name: Build & push runner image
         if: matrix.ENV == 'qemu' && inputs.upload_artifacts
         timeout-minutes: 30
-        uses: docker/build-push-action@v5
+        uses: docker/build-push-action@v6
         with:
           context: .
           push: true