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 :)