Install of raring kernel on precise

Docker works best on the 3.8 kernel.
Precise comes with a 3.2 kernel, so we need to upgrade it.

Stolen from http://docs.docker.io/en/latest/installation/ubuntulinux/

---
I am not sure if this is the approach that you would wish to take or not, however it fixed the following issue I ran into. 
On my 12.04 installation using the raring kernel, the module was trying to install the headers of the raring kernel however it couldn't find them.

```sh
Error: Execution of '/usr/bin/apt-get -q -y -o DPkg::Options::=--force-confold install linux-image-extra-3.8.0-31-generic' returned 100: Reading package lists...
Building dependency tree...
Reading state information...
E: Unable to locate package linux-image-extra-3.8.0-31-generic
E: Couldn't find any package by regex 'linux-image-extra-3.8.0-31-generic'

Error: /Stage[main]/Docker::Install/Package[linux-image-extra-3.8.0-31-generic]/ensure: change from purged to present failed: Execution of '/usr/bin/apt-get -q -y -o DPkg::Options::=--force-confold install linux-image-extra-3.8.0-31-generic' returned 100: Reading package lists...
Building dependency tree...
Reading state information...
E: Unable to locate package linux-image-extra-3.8.0-31-generic
E: Couldn't find any package by regex 'linux-image-extra-3.8.0-31-generic'
```

:) Thanks for the module .

Danny