Enable Postfix to receive external mail

[ikselven]

Postfix should be enabled to receive external mail, i.e. accept mail destined towards users of the groupware that is sent from outside the groupware.

This requires adapting the groupware's docker-compose deployment as well as changes to Maintaina's Postfix image

[ralflang]

We need to re-think this. Email exists in a quite rigid ecosystem. Other mail servers expect a valid reverse DNS lookup at the very least. AWS blocks the SMTP port until you create a ticket with a valid reason why you need SMTP and how you ensure there won't be abuse. Some other servers greylist your server when it is too new or does not provide SPF or DMARC signatures.

On the deployment level, running a fully featured email server in public must be opt-in with a sane template/default. On the container level we need to remove any blockers for this, i.e. move any communication from unix sockets to tcp sockets with appropriate container to container comms.