fix: Only deploy nginx on Swarm managers

docker/test/stack-config/nginx.yaml

@@ -3,6 +3,9 @@ services:
     image: ghcr.io/2martens/docker-nginx:test
     deploy:
       mode: global
+      placement:
+        constraints:
+          - node.role == manager
     ports:
       - target: 80
         published: 80