adding placement restraint to add visualizer, must run on a manager

Author: ManoMarks

docker-stack.yml

@@ -67,6 +67,8 @@ services:
         delay: 10s
         max_attempts: 3
         window: 120s
+      placement:
+        constraints: [node.role == manager]
 
   visualizer:
     image: dockersamples/visualizer:stable