Edits and fixes to Dockerfile Best Practices based on review feedback.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Edits and fixes to Dockerfile Best Practices based on review feedback.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Merge pull request #8222 from fredlf/adding_official-repo-guidelines

Edits and fixes based on review.