help contributors be more productive

certainly saved me time after I found this in hack/make.sh :)

Docker-DCO-1.1-Signed-off-by: Tim Ruffles <oi@truffles.me.uk> (github: github_handle)
Merge pull request #4323 from timruffles/patch-2

help contributors be more productive
Update mkimage-yum to not read /etc/redhat-release from the host

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4348 from tianon/fix-yum-tagging

Update mkimage-yum to not read /etc/redhat-release from the host