allows os-level deps


Improvement to allow os-level deps to be introduced to the container. Typical use-case for this is when you have gems in a Ruby application that require native extensions.

To utilise this functionality you just need to create a `.apt-packages` file and add packages to be installed on
separate lines.
Because the original `apt-get` is a `RUN` it will execute when this image is built. That could be a long way from when the `ONBUILD` is triggered given Docker's container caching, so I think it's best to ensure things are up to date.

![](http://www.quickmeme.com/img/b1/b1279c28446b064a611427b0056df440185c2981c48ce6b07ce119c19097f6e8.jpg)