Add multi-stage builds support
Add multi-stage builds support

Signed-off-by: Fabio Bertinatto <fbertina@redhat.com>

Closes: #495
Approved by: nalind

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