Dockerfiletut

Added the dockerfile tutorial to the docs page @fredlf. Let me know if you have any other suggestions or changes!

closes https://github.com/docker/docker/issues/7206

From the Create Images page:
![screen shot 2014-10-06 at 10 55 10 am](https://cloud.githubusercontent.com/assets/6186377/4530635/937ee11a-4d82-11e4-9f4a-9a0c11a72ea2.png)

CTAs:
![screen shot 2014-10-06 at 11 01 28 am](https://cloud.githubusercontent.com/assets/6186377/4530662/db4690b0-4d82-11e4-8e40-9f6f193d2f35.png)
![screen shot 2014-10-06 at 10 55 20 am](https://cloud.githubusercontent.com/assets/6186377/4530665/e07615c4-4d82-11e4-9b92-ae149b4b7897.png)

level 1:
![screen shot 2014-10-06 at 10 55 31 am](https://cloud.githubusercontent.com/assets/6186377/4530670/eb13d9ee-4d82-11e4-9d3b-7106f76dbd3d.png)
![screen shot 2014-10-06 at 10 58 12 am](https://cloud.githubusercontent.com/assets/6186377/4530671/f1d54be6-4d82-11e4-921c-dcfbaf6e1911.png)

level 2:
![screen shot 2014-10-06 at 10 58 19 am](https://cloud.githubusercontent.com/assets/6186377/4530674/fe821694-4d82-11e4-9525-6cabf6a07742.png)
![screen shot 2014-10-06 at 10 58 28 am](https://cloud.githubusercontent.com/assets/6186377/4530677/00997b48-4d83-11e4-80b9-ee0f820b08c3.png)
![screen shot 2014-10-06 at 10 58 32 am](https://cloud.githubusercontent.com/assets/6186377/4530680/02ae0796-4d83-11e4-95d1-9f8c2ab9ac95.png)

Signed-off-by: Nathan Hsieh <hsieh.nathan@gmail.com>