Don't push attestation of Docker images to the registry

Generates additional entries in the list of image versions, some
are labelled making it impossible for actions/delete-package-version in
its current state to automatically remove them.

https://github.com/actions/attest-build-provenance/issues/71#issuecomment-2108140285

.github/workflows/publish-docker.yml

@@ -52,7 +52,7 @@ jobs:
         with:
           subject-name: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
           subject-digest: ${{ steps.push.outputs.digest }}
-          push-to-registry: true
+          push-to-registry: false
 
       - name: Delete oldest Docker images
         uses: actions/delete-package-versions@v5