Skip pushing containers on forks (#24106)

.github/workflows/build-image.yml

@@ -63,7 +63,7 @@ jobs:
           platforms: linux/amd64,linux/arm64
           provenance: false
           builder: ${{ steps.buildx.outputs.name }}
-          push: ${{ github.event_name != 'pull_request' }}
+          push: ${{ github.repository == 'mastodon/mastodon' && github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           cache-from: type=gha