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.
@DrSarez can you remove the `{}` from the `GITLAB_DATA_DIR` variable here. I have seen issue with such syntax before.