switch to sameersbn/debian:jessie.20140918 baseimage
do not install the build-essential metapackage, cherrypick what we want
moved dev package installation to install script

the dev packages are purged at the end of the gem installation as these
packages are not required anymore.
use package `ruby-dev`
replace `--no-ri --no-rdoc` with `--no-document` in gem install command
introduced the variable GITLAB_BACKUP_DIR with whom users can determine the containers backup location. this adresses an issue of docker, where docker gets in trouble with mounting subdirectorys of already mounted directorys. thus the backup directory could be set to a location outside the GITLAB_DATA_DIR.

removed braces from within the variable expansion of GIT_BACKUP_DIR

Update README.md

Added documentation for the new GITLAB_BACKUP_DIR variable.