portainer: its data directory is placed, not stated

The mesh resolves it to <root>/portainer/data — where the 16M of
endpoints and users already sit. A textual no-op on this node.

modules/portainer/module.json

@@ -29,7 +29,6 @@
     {
       "id": "data",
       "type": "directory",
-      "path": "/var/lib/portainer/data",
       "mode": "0700"
     },
     {
@@ -42,7 +41,7 @@
         "9443:9443"
       ],
       "volumes": [
-        "/var/lib/portainer/data:/data",
+        "${dir:data}:/data",
         "/var/run/docker.sock:/var/run/docker.sock"
       ]
     },