JupyterHub ignores umask

I'm using the JupyterHub on a docker container.
I set the umask for a specific user, but it is not applied for any file created on jupyter.
I've tried with:
docker run ~ -e NB_UMASK='0002'
and inside the jupyterhub_config.py:
c.Spawner.environment = {'NB_UMASK': '0002'} # also tried with c.SystemUserSpawner

The below shows umask and two files created on bash and jupyterhub, repectively.

enter image description here

It seems that JupyterHub ignores not only umask but also NB_UMASK.

How can I properly set the permission for each user?

p.s. These are all of the modifications of jupyterhub_config.py:

c.Authenticator.admin_users = {'kslee'}  
c.Spawner.default_url = '/lab'  
c.Spawner.notebook_dir = '/docker_volumn'  
c.JupyterHub.ip = '0.0.0.0'  
c.JupyterHub.port = 9020  
c.Spawner.environment = {'NB_UMASK': '0002'} # tried both with/without this line