Run builds in customized docker image

This MR introduces customized build image (based on ruby:2.2 and bin/prepare script). Image will be used to make builds a little faster.

@grzesiek This is a draft (however almost working 😉). It'll be rebased on top of latest changes in !1 (merged) and I need to adjust it a little more(already rebased). But it already shows how it would be used :)