Tweaks to Dockerfile tutorial

Made a few tweaks to Dockerfile tutorial links and removed some cruft from the tutorial itself.

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

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Merge pull request #8762 from fredlf/tutorial-tweaks

Tweaks to Dockerfile tutorial