text: Title: AttributeError: module 'os' has no attribute 'geteuid'
  Body: ### 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
  
  