Permissions worked. Subject-name might be off.

.github/workflows/docker-image-ci.yml

@@ -63,6 +63,6 @@ jobs:
         uses: actions/attest-build-provenance@v1
         id: attest
         with:
-          subject-name: index.docker.io/xnat/mice-image-splitter
+          subject-name: ${{ steps.push.outputs.name }}
           subject-digest: ${{ steps.push.outputs.digest }}
           push-to-registry: true