Ghost Docker Container: SMTP

I am used the Docker install. How/where to I go to setup SMTP so that I can invite other contributors?

I do have an SMTP provider but not sure where to set these up.

I will try translating the above link later to see if that gives me any pointers.

As the part I need is for registered users, that does not help me., but this might: Ghost SMTP Integration Guide | Mailazy Docs

Will try later.