text: Title: FileNotFoundError: [Errno 2] No such file or directory: 'git': 'git'
  Body: I'm trying to run pytest in docker container. I always use pytest in my projects with docker.
  Is the `.git` folder missing in the container?
  
  This is my `.dockerignore`:
  ```
  
  .*
  !.coveragerc
  !.env
  !.pylintrc
  
  ```
  
  This is traceback:
  ```
  
  INTERNALERROR> Traceback (most recent call last):
  INTERNALERROR>   File "/usr/local/lib/python3.6/site-packages/_pytest/main.py", line 174, in wrap_session
  INTERNALERROR>     config._do_configure()
  INTERNALERROR>   File "/usr/local/lib/python3.6/site-packages/_pytest/config/__init__.py", line 593, in _do_configure
  INTERNALERROR>     self.hook.pytest_configure.call_historic(kwargs=dict(config=self))
  INTERNALERROR>   File "/usr/local/lib/python3.6/site-packages/pluggy/hooks.py", line 306, in call_historic
  INTERNALERROR>     res = self._hookexec(self, self.get_hookimpls(), kwargs)
  INTERNALERROR>   File "/usr/local/lib/python3.6/site-packages/pluggy/manager.py", line 67, in _hookexec
  INTERNALERROR>     return self._inner_hookexec(hook, methods, kwargs)
  INTERNALERROR>   File "/usr/local/lib/python3.6/site-packages/pluggy/manager.py", line 61, in <lambda>
  INTERNALERROR>     firstresult=hook.spec.opts.get("firstresult") if hook.spec else False,
  INTERNALERROR>   File "/usr/local/lib/python3.6/site-packages/pluggy/callers.py", line 208, in _multicall
  INTERNALERROR>     return outcome.get_result()
  INTERNALERROR>   File "/usr/local/lib/python3.6/site-packages/pluggy/callers.py", line 80, in get_result
  INTERNALERROR>     raise ex[1].with_traceback(ex[2])
  INTERNALERROR>   File "/usr/local/lib/python3.6/site-packages/pluggy/callers.py", line 187, in _multicall
  INTERNALERROR>     res = hook_impl.function(*args)
  INTERNALERROR>   File "/usr/local/lib/python3.6/site-packages/pytest_picked/plugin.py", line 45, in pytest_configure
  INTERNALERROR>     picked_files, picked_folders = mode.affected_tests()
  INTERNALERROR>   File "/usr/local/lib/python3.6/site-packages/pytest_picked/modes.py", line 12, in affected_tests
  INTERNALERROR>     raw_output = self.git_output()
  INTERNALERROR>   File "/usr/local/lib/python3.6/site-packages/pytest_picked/modes.py", line 31, in git_output
  INTERNALERROR>     output = subprocess.run(self.command(), stdout=subprocess.PIPE)
  INTERNALERROR>   File "/usr/local/lib/python3.6/subprocess.py", line 403, in run
  INTERNALERROR>     with Popen(*popenargs, **kwargs) as process:
  INTERNALERROR>   File "/usr/local/lib/python3.6/subprocess.py", line 709, in __init__
  INTERNALERROR>     restore_signals, start_new_session)
  INTERNALERROR>   File "/usr/local/lib/python3.6/subprocess.py", line 1344, in _execute_child
  INTERNALERROR>     raise child_exception_type(errno_num, err_msg, err_filename)
  INTERNALERROR> FileNotFoundError: [Errno 2] No such file or directory: 'git': 'git'
  
  ```