So what I ended up doing was:
- Verify example.com with Mailgun
- Install Discourse at www.example.com
- The email arrived, but from unusual noreply@www.example.com instead of noreply@example.com. So I set the email via
docker exec -it app rails r 'puts SiteSetting.notification_email = "noreply@example.com"'(in this case probably an easier way would be just to finish the admin registration and then set email in the /admin UI).