do not install checkinstall package
rebase image on sameersbn/debian:jessie.20140918 base image
do not install the build-essential metapackage, cherrypick what we want
docker readability updates
purge development packages after install
replace `--no-ri --no-rdoc` with `--no-document` in gem install command
Make it possible to set URL and Token of CI via environment

Instead of doing interactive setup you can now set
CI_SERVER_URL and REGISTRATION_TOKEN environment variables.