Mail-receiver log errors after 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

https://github.com/discourse/mail-receiver/pull/12 @david This should fix both issues mentioned in https://meta.discourse.org/t/self-hosted-mail-receiver-update-following-lets-encrypt-root-certificate-change/204924/6?u=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).