Use namshi/smtp as SMTP container

[oliv3]

Replace Postfix Dockerfile with namshi/smtp.
This image is simpler to use and configure ("less code, less bugs" :) ).
Warning: it's a backport from one of my other projects, so had no time to thoroughly test it here, but it should be pretty straight-forward.

[terrillmoore]

I'm a little nervous about exim as the base, as I've never used it; but I guess there's a first time for everything. However, looks like you've removed the ability to relay mail, which I use. And I can't tell if any of the modern mail anti-spam features are used (names escape me, but there's a DNS-based thing, and a signing thing that's more advanced). That's why I use a relay, the relay server takes care of all that so I don't have to duplicate.

[terrillmoore]

Also, same comments about updating docs.

[oliv3]

Ok, didn't know for the relay part (consider this as a WIP). According to the documentation:

The container accepts RELAY_NETWORKS environment variable which MUST start with : e.g :192.168.0.0/24 or :192.168.0.0/24:10.0.0.0/16.

So I can add this back. As for the docs, saw you were updating them, decided to wait till it's done.

[oliv3]

You can also use any generic SMTP server with authentication as smarthost.
You need to set SMARTHOST_ADDRESS, SMARTHOST_PORT (connection parameters), SMARTHOST_USER, SMARTHOST_PASSWORD (authentication parameters), and SMARTHOST_ALIASES: this is a list of aliases to puth auth data for authentication, semicolon separated.

[oliv3]

@terrillmoore Just added configuration variables in case you would want to test the relay feature.