Error: invalid mount target, must be an absolute path: SwarmSpawner

hello I am trying to setup jupyterhub with docker. I can get service started for jupyterhub and nginx (reverse proxy). When I login thought the jupyter-notebook server fails to start. The jupyter/datascience-notebook service starts, but fails to create a docker container. The related service tag shows the error :

ID                          NAME                  IMAGE                                 NODE                             DESIRED STATE       CURRENT STATE             ERROR                                                            PORTS
ral7xr7nxhbqjvvh723ljqbkh   jupyter-alvin.1       jupyter/datascience-notebook:latest   Ready               Rejected 3 seconds ago    "invalid mount target, must be an absolute path: SwarmSpawner"

Where in the file would a set an “absolute path” for the mount target? would this be the NOTEBOOK_DIR? here is the what I think the relevant section from my file:

notebook_dir = os.environ.get('NOTEBOOK_DIR') or '/home/jovyan/work'
c.SwarmSpawner.notebook_dir = notebook_dir

mounts = [{'type': 'volume',
           'source': 'jupyterhub-user-{username}',
           'target': notebook_dir,
           'no_copy' : True,
           'driver_config' : {
              'name' : 'local',
              'options' : {
             'type' : 'nfs4',
             'o' : 'addr=jupterhubserver,rw',
             'device' : ':/var/nfs/{username}/'

If it is NOTEBOOK_DIR . what should I set that to? Currently I am mounting a docker volume to /var/nfs with in the jupyterhub container.
any help would be greatly appreciated.