Use busybox in fixtures, instead of ubuntu

I develop exclusively inside docker containers, because I like to make sure my development environments are both disposable and reproducible. This includes hacking on fig, by running [docker inside docker](https://github.com/d11wtq/fig-dev-docker).

Because my dev environment begins with a clean slate (i.e. no docker images), using ubuntu as a text fixture is a bit heavyweight. Quite often I'm limited to using a 3G connection tethering on my phone too. Switching to busybox reduces the bandwidth overhead to all of 2.5MB instead of 200MB. Given you only need /bin/sleep, this should be sufficient. It also speeds up the tests, as the initial pull has way less work to do.