code: def _get_env_vars_value(filename):
          
          if not filename:
              return None
  
          # Try to read the file and parse it as JSON
          try:
  
              with open(filename, 'r') as fp:
                  return json.load(fp)
  
          except Exception as ex:
              raise InvokeContextException("Could not read environment variables overrides from file {}: {}".format(
                                           filename,
                                           str(ex)))
docs: If the user provided a file containing values of environment variables, this method will read the file and
          return its value
  
          :param string filename: Path to file containing environment variable values
          :return dict: Value of environment variables, if provided. None otherwise
          :raises InvokeContextException: If the file was not found or not a valid JSON