[Compose] Add missing var

docker-compose.yml

@@ -245,7 +245,7 @@ services:
         - HTTPS_PORT=${HTTPS_PORT:-443}
         - HTTP_PORT=${HTTP_PORT:-80}
         - MAILCOW_HOSTNAME=${MAILCOW_HOSTNAME}
-        - IPV4_NETWORK=
+        - IPV4_NETWORK=${IPV4_NETWORK:-172.22.1}.254
       volumes:
         - ./data/web:/web:ro
         - ./data/conf/rspamd/dynmaps:/dynmaps:ro