Add ONBUILD installing required libraries

[vlcinsky]

Installing lxml, pyyaml, pyzmq and couple of other python packages will currently fail, as they require some libraries in system to be installed.

One solution is to prepare the basic python image with all of them already installed:

$ apt-get install python-dev libxml2-dev libxslt-dev lib32z1-dev libyaml-dev libzmq-dev

Another solution would be to add another ONBUILD line before calling $ pip install -r requirements which would call a script (e.g. called pre-install-dependencies.sh) installing those dependencies.

[2xyo]

👍

[clementmangin]

I made a pull request addressing this issue, you can check it out here: #2