Setting up and testing email?

I have a working Gitlab CE install from the omnibus package. The last part I need to configure is email (so that users can set their password, sign up etc).

On the same box as my Gitlab Docker container, I have a mail server built using this container. I know my mail server works, because I can type something in through telnet and have it sent via postfix.

I’m trying to find out what to configure so that Gitlab talks to postfix.
When I run gitlab-rake gitlab:check it fails during the IMAP settings check, but I’m not sure what this means…

What can I do next to debug this? Where can I find out the right settings for this (hopefully simple) problem?
Once I finish configuration, is there a generic ‘test email from gitlab’ option somewhere? Or do I just use the user-facing email features?