Fix signing of docker image (#29)

.github/workflows/docker-publish.yml

@@ -134,7 +134,7 @@ jobs:
           COSIGN_EXPERIMENTAL: "true"
         # This step uses the identity token to provision an ephemeral certificate
         # against the sigstore community Fulcio instance.
-        run: echo "${{ steps.meta.outputs.tags }}" | xargs -I {} cosign sign {}@${{ steps.build-and-push.outputs.digest }}
+        run: echo "${{ steps.meta.outputs.tags }}" | xargs -I {} cosign sign {}@${{ steps.docker_build_runtime_cross.outputs.digest }}
 
       - name: git commit README.md 
         id: commit