Create `postfix` container to handle mail relaying

[ssoriche]

Add a container that sets up a postfix mail relay for use when sending mail from any of the other containers.

[waterkip]

Do you want to send out real mails to the world or do you want to test mail functionality? For the latter I have found the mailhog image to be an excellent tool. It allows to have a dummy mailserver with a UI to look at the mails you send out.