build: define noninteractive debian frontend
README: removed the bash instruction from code blocks
build: moved unzip to essential tools list
build: apply upstart fix from https://github.com/dotcloud/docker/issues/1024
install: defined macros for gitlabhq and gitlab-shell versions
install: clone gitlab and shell repositories if local source tarball does not exist
cleanup: fix indentation
build: add extra ppa's before installing image specific packages.
build: combine apt-install commands