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)
Convert api/server.go to also use the handy user package

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #4335 from tianon/what-the-etc-group

Convert api/server.go to also use the handy user package
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
Uncover errors on existing tests regarding ADD cache and file metadata

Docker-DCO-1.1-Signed-off-by: Abel Muiño <amuino@gmail.com> (github: amuino)
Fix ADD cache issues by skipping file headers (only sum the content).

Docker-DCO-1.1-Signed-off-by: Abel Muiño <amuino@gmail.com> (github: amuino)
Reformatted source code

Docker-DCO-1.1-Signed-off-by: Abel Muiño <amuino@gmail.com> (github: amuino)