new 2020 image builds with 'start' config failing to launch on binder or jupyterhub

[scottyhq]

2020.02.03-0853dbb and latest tags are not launching on binder or jupyterhubs, seems related to recent changes enabling start scripts for onbuild images:
https://github.com/pangeo-data/pangeo-stacks/commits/master/onbuild/r2d_overlay.py

Here is the traceback from pod logs that get stuck in CrashLoopBackOff

Traceback (most recent call last):
  File "/usr/local/bin/r2d_overlay.py", line 167, in <module>
    main()
  File "/usr/local/bin/r2d_overlay.py", line 163, in main
    start()
  File "/usr/local/bin/r2d_overlay.py", line 135, in start
    ['/bin/bash', '-c', command], preexec_fn=applicator._pre_exec
  File "/srv/conda/envs/notebook/lib/python3.7/subprocess.py", line 358, in check_call
    retcode = call(*popenargs, **kwargs)
  File "/srv/conda/envs/notebook/lib/python3.7/subprocess.py", line 339, in call
    with Popen(*popenargs, **kwargs) as p:
  File "/srv/conda/envs/notebook/lib/python3.7/subprocess.py", line 800, in __init__
    restore_signals, start_new_session)
  File "/srv/conda/envs/notebook/lib/python3.7/subprocess.py", line 1552, in _execute_child
    raise child_exception_type(err_msg)
subprocess.SubprocessError: Exception occurred in preexec_fn.

pinging @rsignell-usgs who encountered this, also @jhamman, @yuvipanda, and @scollis for possible fixes

[rabernat]

Could this be the cause of pangeo-data/pangeo-cloud-federation#523?

[scottyhq]

closed by #118