So if you run anything other than Ubuntu inside Docker, this is useless because the steps to build your own aren't outlined.

I find Docker to be horribly counter-intuitive and ass-backwards anyway, so not much harm done there as people are in general better off with something else entirely (plain lxc, libvirt, virtualbox, xen, openvz...). I recommend to steer away from it at least until 1.0 is out.

EDIT: I put it in my .plan to build a better BusyBox image aimed at running statically compiled programs with minimal baggage, but I'm not sure when I'll get a round tuit*

*: http://i.ebayimg.com/00/s/NDgwWDY0MA==/z/z-4AAOxyUrZSr82N/$_...