Add contrib/mkimage-centos.sh back (from #1621), and associated documentation link
fix panic with wrong dockercfg file
Merge pull request #2046 from dotcloud/2030-dockercfg_panic_err-fix

- Runtime: fix panic with wrong dockercfg file
Merge pull request #2039 from tianon/mkimage-centos

Add contrib/mkimage-centos.sh back (from #1621), and associated documentation link