changed dockerfile back button to look nicer

Updated back button from:

![screen shot 2014-10-27 at 1 45 38 pm](https://cloud.githubusercontent.com/assets/6186377/4799165/427e77b4-5e1a-11e4-8ed1-b442654ede1e.png)

to:

![screen shot 2014-10-27 at 1 45 24 pm](https://cloud.githubusercontent.com/assets/6186377/4799168/4b8dfe7e-5e1a-11e4-9594-804e3cba0af7.png)