Replace them with:
* containers' hostnames
* gateway's IP address for default SMTP_HOST
* Core: when SMTP_HOST undef, echo the default value
* docker-compose: add port mapping
* docker-compose: drop network driver conf
* Exim macro: use localhost as LMTP host
* Update README.md
* docker-compose: Limit port mapping to loopback
* Update other docker-compose example files
* Remove the secret key.
* Use the latest tag for postorius.
* Use the latest django-mailman3 that supports Python 3.
* Use minimum version on Django-mailman3.
* Fix import errors.