Document how to pass ENV vars to the user's shell

As this is a somewhat common question.

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
Merge pull request #8005 from SvenDowideit/document-sshd-env-vars

Document how to pass ENV vars to the user's shell