Postfix default configuration can easily be overrided providing a `config/postfix-main.cf` at postfix format.
This can be used to also add configuration that are not in out default configuration.

Each line in the provided line will be loaded into postfix.

Have a look to the code for more information: 
https://github.com/tomav/docker-mailserver/blob/master/target/start-mailserver.sh#L196-L206