Use imagebuilder's definition of "scratch"
Use imagebuilder's definition of "scratch"

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

Closes: #59
Approved by: rhatdan

TomSweeneyRedHat/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,rhatdan/buildah,nalind/buildah,ipbabble/buildah,rhatdan/buildah,nalind/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,projectatomic/buildah,rhatdan/buildah,nalind/buildah-1,nalind/buildah-1,ipbabble/buildah,projectatomic/buildah