Docker syntax

Use shell highlighter for Dockerfiles.

Before:

![image](https://f.cloud.github.com/assets/1050/865751/ca3ee214-f643-11e2-9e5f-671e68995a4c.png)

After:

![image](https://f.cloud.github.com/assets/1050/865747/a6305b14-f643-11e2-92c7-081b1fc78c84.png)