Add path to catnip Dockerfile to action

Author: jochenehret

.github/workflows/build-catnip-docker-image.yml

@@ -35,6 +35,7 @@ jobs:
         uses: docker/build-push-action@f2a1d5e99d037542a71f64918e516c093c6f3fc4
         with:
           context: .
+          file: ./assets/catnip/Dockerfile
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}