Doc environment variables 593

fixes #593 
fixes #460 
Use `ssh -t` here otherwise you get very little feedback from the docker command.
```
$ ssh root@172.16.255.250 docker pull mysql:5.6.17
Pulling repository mysql














```

Vs

```
$ ssh -t root@172.16.255.251 docker pull mysql:5.6.17
Pulling repository mysql
36e732ca2610: Download complete 
511136ea3c5a: Download complete 
1e8abad02296: Download complete 
f106b5d7508a: Download complete 
bd80da5e6905: Download complete 
b0404087597a: Download complete 
23aa80dafcd4: Download complete 
ce7afff7ddce: Download complete 
64480a6e5ec9: Download complete 
8c971c465b8e: Download complete 
d388127074e1: Download complete 
f61167f35d6a: Download complete 
ef8b6c2f3e7f: Download complete 
Connection to 172.16.255.251 closed.

```