[Users] how to set main IP address for a container

[Rene C.]

I've noticed that when I have multiple IP addresses bound to a
container, the last added (highest venet0:x number) seems to always
ends up being the "main" ip address for the container.

This means when I add another IP to the server, the main ip for the
server changes.

By main IP I mean the IP address used for outgoing traffic, the one
that a receiving mail server or ssh server would see the incoming
traffic originating from.

How can I change this behavior and set a fixed venet0:x as the "main" IP?