initial stab at writing a use/ambassador-pattern howto

@Lennie @tianon @shykes @metalivedev

please critique away, I was learning as I went :)

I think the ``svendowideit/ambassador`` image should be in stackbrew

also, https://index.docker.io/_/docker-ut/ does not match the ``./contrib/mkimage-unittest.sh`` image - which added socat months ago - so that should probably also be updated (and renamed to socat??)

 (Issue #2485)