Changed add to group command to include -a (append) flag, for safety. In response to customer email.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Merge pull request #6741 from fredlf/suse-install-fix

Changed add to group command to include -a (append) flag, for safety. In...