Add "SteamOS" support to mkimage-debootstrap (requires a debootstrap script like https://github.com/tianon/gentoo-overlay/blob/master/dev-util/debootstrap-valve/files/scripts/alchemist)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4288 from tianon/mkimage-steamos

Add "SteamOS" support to mkimage-debootstrap
Engine: add tests for ParseJob()

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Engine: tiny cosmetic fix

To make @creack happy :)

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)