Change single-dash flag usage to double-dash

The single-dash long-form flag usage is deprecated.

Docker-DCO-1.1-Signed-off-by: Nathan LeClaire <nathanleclaire@gmail.com> (github: nathanleclaire)
Merge pull request #5949 from nathanleclaire/patch-1

Change single-dash flag usage to double-dash
Squashed for pull request
Updated documentation to include CentOS installation instructions.
Extraneous line removal, description change

Docker-DCO-1.1-Signed-off-by: Jim Perrin <jperrin@centos.org> (github: jimperrin)