Add explicit buildx output to docker

Author: abkfenris

.github/workflows/push.yml

@@ -40,6 +40,7 @@ jobs:
         tags: axiom/docker-erddap:latest
         cache-from: type=local,src=${ BUILDX_CACHE }
         cache-to: type=local,dest=${ BUILDX_CACHE }
+        outputs: type=docker
 
     - name: Run Docker Image in Background
       run: docker run -d -p 8080:8080 axiom/docker-erddap:latest