AttributeError: module 'os' has no attribute 'geteuid'

Bug description

Expected behaviour

Expect to produce docker image.

Actual behaviour

Traceback (most recent call last):
  File "c:\appdata\local\continuum\anaconda3\lib\site-packages\traitlets\traitlets.py", line 528, in get
    value = obj._trait_values[self.name]
KeyError: 'user_id'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "c:\appdata\local\continuum\anaconda3\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "c:\appdata\local\continuum\anaconda3\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\AppData\Local\Continuum\anaconda3\Scripts\repo2docker.exe\__main__.py", line 9, in <module>
  File "c:\appdata\local\continuum\anaconda3\lib\site-packages\repo2docker\__main__.py", line 345, in main
    r2d = make_r2d()
  File "c:\appdata\local\continuum\anaconda3\lib\site-packages\repo2docker\__main__.py", line 300, in make_r2d
    if r2d.user_id == 0 and not r2d.dry_run:
  File "c:\appdata\local\continuum\anaconda3\lib\site-packages\traitlets\traitlets.py", line 556, in __get__
    return self.get(obj, cls)
  File "c:\appdata\local\continuum\anaconda3\lib\site-packages\traitlets\traitlets.py", line 535, in get
    value = self._validate(obj, dynamic_default())
  File "c:\appdata\local\continuum\anaconda3\lib\site-packages\repo2docker\app.py", line 206, in _user_id_default
    return os.geteuid()
AttributeError: module 'os' has no attribute 'geteuid'

How to reproduce

1. On a Windows 10 PC, open the Anaconda Command Prompt
2. Run repo2docker https://github.com/matthewolckers/fbr2020

• OS: Windows 10
• Docker version: 19.03.8,
• repo2docker version 0.11.0