docker: Don't push to the registry for Pull Requests

This requires more thought on how to handle secrets, and whether to just
omit it permanently.

.github/workflows/ci-docker.yml

@@ -45,13 +45,14 @@ jobs:
         with:
           context: .
           platforms: linux/amd64,linux/arm64,linux/arm/v7,windows/amd64
-          push: true
+          push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
           pull: true
           cache-from: type=gha, scope=${{ github.workflow }}
           cache-to: type=gha, scope=${{ github.workflow }}
       - name: Generate build provenance attestation
+        if: ${{ github.event_name != 'pull_request' }}
         uses: actions/attest-build-provenance@v2
         with:
           subject-name: ghcr.io/${{ github.repository }}