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.