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.

services/rspamd/conf/worker-controller.inc

@@ -1,4 +1,4 @@
 type = "controller";
 bind_socket = "*:11334";
 password = "mailu";
-secure_ip = "{% if POD_ADDRESS_RANGE %}{{ POD_ADDRESS_RANGE }}{% else %}{{ FRONT_ADDRESS }}{% endif %}";
+secure_ip = "{{ POD_ADDRESS_RANGE or SUBNET }}";