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

Also, dockerfile edits so that makedocs works again.

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)

Fixed small line break/punctuation error.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)