Mail-receiver logfouten na letsencrypt update

[md-misko]

https://github.com/discourse/mail-receiver/pull/12

Mail-receiver loses IP to hostname resolution after recent upgrade and prevents anti-spam measures from working

@david This should fix both issues mentioned in Self-hosted mail-receiver update following Let's Encrypt root certificate change - #6 by md-misko Please evaluate whether removing postfix from the chroot jail introduces any security issues (IMO it shouldn’t, as postfix is already isolated inside a dedicated docker image).