adding a placement constraint on visualizer so it runs on a manager node

docker-stack.yml

@@ -77,10 +77,13 @@ services:
     stop_grace_period: 1m30s
     volumes:
       - "/var/run/docker.sock:/var/run/docker.sock"
+    deploy:
+      placement:
+        constraints: [node.role == manager]
 
 networks:
   frontend:
   backend:
 
 volumes:
-  db-data:
+  db-data: