initial creation, adapted from https://github.com/sameersbn/docker-gitlab-ci
use dockerfile from https://github.com/sameersbn/docker-gitlab so that most of the cache can be shared between the images
build: define noninteractive debian frontend
build: apply upstart fix from https://github.com/dotcloud/docker/issues/1024
added unzip to essential packages list
packages: install ruby2.0 via ppa instead of compiling from source
removed unused packages
combine apt-install commands
README: removed the bash instruction from code blocks to fix markdown rendering at index.docker.io
updated changelog
README: use latest tag in docker commands
dockerfile: rebuild apt cache so that the libyaml vulnerability is patched.
rebuild apt cache so that libssl heartbleed vulnerability is patched