Add provenance:false to prevent unknown archs

Even if everything is working as expected, for each run an additional image
related to an "unknow" architecture is created. This, extracted from [1],
should avoid this.

[1] docker/build-push-action#820

.github/workflows/main.yml

@@ -35,6 +35,8 @@ jobs:
       with:
         context: .
         push: ${{ github.event_name != 'pull_request' }}
+        # Check https://github.com/docker/build-push-action/issues/820
+        provenance: false
         platforms: linux/amd64,linux/arm64
         tags: |
            ghcr.io/mmul-it/kpa-marp-pandoc:latest