Move the Mailu Docker network to a fixed subnet.

This will make network configuration and host based authentication
more robust, across different deployment platforms.
The options `RELAYNETS` and`POD_ADDRESS_RANGE` are kept for compatibility.
However, their usage have become optional.

docs/compose/docker-compose.yml

@@ -104,3 +104,11 @@ services:
     image: mailu/fetchmail:$VERSION
     restart: always
     env_file: .env
+
+  networks:
+    default:
+      driver: bridge
+      ipam:
+      driver: default
+      config:
+        - subnet: $SUBNET