chore: fix portainer version

Author: l4rm4nd

examples/portainer/docker-compose.yml

@@ -2,7 +2,7 @@ version: '3'
 
 services:
   portainer:
-    image: portainer/portainer-ee:latest # or use community edition via portainer/portainer-ce
+    image: portainer/portainer-ee:2.21.0-alpine # or use community edition via portainer/portainer-ce
     container_name: portainer-ee # may rename to portainer-ce if community edition is used
     restart: unless-stopped
     volumes: