Fetch the "busybox" image source so we can build locally instead of pulling during the integration tests

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
allow dot in repo name

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
contrib: mkimage script for alpine linux

Docker-DCO-1.1-Signed-off-by: Eivind Uggedal <eivind@uggedal.com> (github: uggedal)
Merge pull request #5202 from vieux/allow_dot_repo

Allow dot in repo name
Merge pull request #5181 from uggedal/mkimage-alpine

contrib: mkimage script for alpine linux
Merge pull request #5201 from tianon/build-busybox

Fetch the "busybox" image source so we can build locally instead of pulling during the integration tests