Override default postfix configuration

The Postfix default configuration can easily be overridden providing a config/postfix-main.cf at postfix format. This can be used to also add configuration that are not in out default configuration. Postfix documentation remains the best place to find configuration options.

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#L205-L215