fix content-type docker copy

Docker-DCO-1.1-Signed-off-by: Victor Vieux <victor.vieux@docker.com> (github: vieux)
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
Merge pull request #4341 from vieux/fix_content_type_copy

fix content-type docker copy