compose: join external storage network

The new asxpio/storage stack owns a Docker network named "storage" carrying
Postgres and MinIO. The app joins it (alongside traefik) so it can reach
postgres:5432 and minio:9000 by service name.

docker-compose.yml

@@ -9,6 +9,7 @@ services:
     env_file: .env
     networks:
       - traefik
+      - storage
     labels:
       - "traefik.enable=true"
       - "traefik.http.routers.asxpio.rule=Host(`asxp.io`) || Host(`www.asxp.io`)"
@@ -20,3 +21,5 @@ services:
 networks:
   traefik:
     external: true
+  storage:
+    external: true