Added warning re: using root for dockerfile root. Fixes #7612

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Fixed a typo and added a bit based on Sven's feedback

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Merge pull request #7633 from fredlf/root_builder_warning

Added warning re: using root for dockerfile root. Fixes #7612