Changed the git user to dokku and fixed old things

Used $DOKKU_ROOT and $DOKKU_VOLUME_PATH instead of fixed literals. This is much more maintainable.
TODO:check bugs and to be able to link same mysql-container to multiple apps
volume dir changed making older dokku incompatible

This uses /var/lib/docker/vfs/dir
instead of older :
/var/lib/docker/volumes
Making it incompatible with older dokku versions