Add missing outputs param to docker build and push action

Author: brendan-kellam

.github/workflows/ghcr-publish.yml

@@ -82,6 +82,7 @@ jobs:
           cache-from: type=gha
           cache-to: type=gha,mode=max
           platforms: ${{ matrix.platform }}
+          outputs: type=image,name=${{ env.REGISTRY_IMAGE }},push-by-digest=true,name-canonical=true,push=true
 
       - name: Export digest
         run: |