Hmm.. I have since found this: https://docs.docker.com/docker-for-mac/networking/#use-cases-and-workarounds. I tried that, but no dice. Not sure if I did everything right. Not sure if postfix is really listening on the alias'd port.
It seems getting OSX dockers to see the host is finnicky. There are other threads about how to access host services from OSX dockers, but I can't seem to really understand.
If I built mattermost via docker-compose, how do I specify network settings, like -network=nat? Will have to understand this a bit better...!