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.

setup/flavors/compose/mailu.env

@@ -25,8 +25,8 @@ SECRET_KEY={{ secret(16) }}
 # PUBLIC_IPV4= {{ bind4 }} (default: 127.0.0.1)
 # PUBLIC_IPV6= {{ bind6 }} (default: ::1)
 
-# Subnet
-SUBNET={{ subnet }}
+# Subnet of the docker network. This should not conflict with any networks to which your system is connected. (Internal and external!)
+SUBNET=192.168.203.0/24
 
 # Main mail domain
 DOMAIN={{ domain }}
@@ -75,9 +75,9 @@ ANTISPAM={{ antispam_enabled or 'none'}}
 # Default: accept messages up to 50MB
 MESSAGE_SIZE_LIMIT={{ message_size_limit or '50000000' }}
 
-# Networks granted relay permissions, make sure that you include your Docker
-# internal network (default to 172.17.0.0/16)
-RELAYNETS={{ relaynets or '172.17.0.0/16' }}
+# Networks granted relay permissions
+# Use this with care, all hosts in this networks will be able to send mail without authentication!
+RELAYNETS=
 
 # Will relay all outgoing mails if configured
 RELAYHOST={{ relayhost }}