In fact, remove the mention of the exported images being in the container
altogether. The images are in the host docker daemon, so it is easier to
export them again than to go fishing around with 'docker cp'.
I'm not sure about using the name 'zettio/weave-build' for the image
here. There's no intent to upload it to dockerhub, so including the
user name 'zettio' seems wrong. On the other hand, I'm not sure what
the meaning of an unslashed image name is supposed to be. Probably it
doesn't matter what a private non-pushed image is called, as long as
it doesn't clash with something.
We know for sure it builds with 1.3.0 and not 1.0.1. I can't be
bothered to check every version inbetween, so let's just say we
require 1.3.0.
Closes#220.