removed default --rm option from example build command

this line gave the false impression that containers are not removed
unless you specify the --rm argument.
Merge pull request #7049 from albers/default-option

removed default --rm option from example build command
Updated Red Hat Enterprise Linux installation instructions for version 7

Docker-DCO-1.1-Signed-off-by: Brian Exelbierd <bex@pobox.com> (github: bcexelbi)