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/.env

@@ -1,6 +1,5 @@
 # Mailu main configuration file
-#
-# Most configuration variables can be modified through the Web interface,
+## Most configuration variables can be modified through the Web interface,
 # these few settings must however be configured before starting the mail
 # server and require a restart upon change.
 
@@ -21,6 +20,9 @@ SECRET_KEY=ChangeMeChangeMe
 BIND_ADDRESS4=127.0.0.1
 BIND_ADDRESS6=::1
 
+# 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=mailu.io
 
@@ -63,9 +65,9 @@ ANTIVIRUS=none
 # Default: accept messages up to 50MB
 MESSAGE_SIZE_LIMIT=50000000
 
-# Networks granted relay permissions, make sure that you include your Docker
-# internal network (default to 172.17.0.0/16)
-RELAYNETS=172.16.0.0/12
+# 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=