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)