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.
Merge branch 'DrSarez-master'