Doc environment variables 593

fixes #593 
fixes #460 
The output looks a little different for me, in particular the *COMMAND*...but I don't suppose it matters too much.
```
(flocker-tutorial)[richard@drax flocker-tutorial]$ ssh root@172.16.255.250 docker ps
CONTAINER ID        IMAGE               COMMAND                CREATED             STATUS              PORTS                    NAMES
7e35a4bc1a13        mysql:5.6.17        /entrypoint.sh mysql   26 seconds ago      Up 25 seconds       0.0.0.0:3306->3306/tcp   mysql-volume-example   
```