Use /flynn/busybox image.

With stackbrew/busybox I first ran into the DNS bug (https://github.com/dotcloud/docker/issues/2468), and the image from 2013-02-04 cannot execute the go binaries for some reason (Not found).

Might as well go with a trusted base image.

Signed-off-by: Michael Elsdörfer <michael@elsdoerfer.com>