SOLUTION (2nd snippet of UPDATE3):
c.DockerSpawner.extra_host_config.update(
binds = {'/sys/fs/cgroup' :
{'bind' : '/sys/fs/cgroup',
'mode' : 'ro' }})
I had already solved the privileged
container dilemma; and integrating that solution with the above solution, yields the (final) complete solution:
c.DockerSpawner.extra_host_config.update(
{"privileged" : True},
binds = {'/sys/fs/cgroup' :
{'bind' : '/sys/fs/cgroup',
'mode' : 'ro' }})