Adds info on how to get env vars from container

Currently the docs just say "let's go into the container and look at the
env vars", but doesn't show how to do it.  It is also not currently
possible to enter the container as it is written in the doc
( container started with app.py and then somehow later on we have a
shell in the same container )

Docker-DCO-1.1-Signed-off-by: Brian Goff <cpuguy83@gmail.com> (github: cpuguy83)
docs: add spotify docker-client link

Docker-DCO-1.1-Signed-off-by: Daniel Norberg <daniel.norberg@gmail.com> (github: danielnorberg)
Merge pull request #6416 from danielnorberg/dano/docs-spotify-client

docs: add spotify docker-client link
Note user name and password in docs on boot2docker

Docker-DCO-1.1-Signed-off-by: Nathan LeClaire <nathan.leclaire@docker.com> (github: nathanleclaire)
Merge pull request #6421 from nathanleclaire/doc_fix

Note user name and password in docs on boot2docker
Fixed whitespace errors
Merge pull request #6386 from cpuguy83/fix_link_docs

Adds info on how to get env vars from container