Though I’d post this here since I found a lot of useful help on this forum and perhaps if I leave a note regarding what I learnt and then implemented then it might help someone else in the future.
I have helped setup a Discourse forum at community.coops.tech using the official Docker install, Postfix doing incoming and outgoing email on the host, with measures to prevent backscatter on Debian Stretch.
The Ansible Playbooks are available at git.coop and the email setup is based on the mail-reciever Docker container plus this pull request (which is now merged) and the Postfix notes for using the host for outgoing email, with an additional Ruby script.
Some of the things in the Playbooks are specific to the Webarchitects environment they have been written for so they probably couldn’t be directly used elsewhere, but if anyone would our help with installing Discourse like this then feel free to contact us.