Fix external network syntax for podman-compose 1.5.0

Old v1 `external: { name: proxy }` is not parsed correctly by
podman-compose — it looked up "frontend" instead of "proxy".
Split into `external: true` + `name: proxy` (Compose v2 format).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -134,7 +134,7 @@ services:
 
 networks:
   frontend:
-    external:
+    external: true
     name: proxy
   rh_net: