Fix docs Dockerfile to install correctly again

Previously we've been getting:

```
Step 6 : RUN pip install Sphinx==1.1.3
 ---> Running in 397eab32f57a
 Wheel installs require setuptools >= 0.8 for dist-info support.
 pip's wheel support requires setuptools >= 0.8 for dist-info support.
 Storing debug log for failure in /.pip/pip.log
 2014/01/13 18:01:34 The command [/bin/sh -c pip install Sphinx==1.1.3] returned a non-zero code: 1
 make: *** [docs] Error 1
```

This fixes that by telling pip not to use sudo to install (since we're already root).

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #3584 from tianon/fix-docs-dockerfile

Fix docs Dockerfile to install correctly again