Update dockerspawner.py support mount type args

[zhangnew]

class Mount(target, source, type='volume', read_only=False, consistency=None, propagation=None, no_copy=False, labels=None, driver_config=None, tmpfs_size=None, tmpfs_mode=None)

see https://docker-py.readthedocs.io/en/stable/api.html#docker.types.Mount

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[manics]

Can you explain why this is needed? args = dict(mount) already incorporates those arguments if they've passed, and if they're not then the Docker defaults are used.

[zhangnew]

Sorry, it's my problem.