Consume container output when it's not parsed

Revert run_command() logic to sending again stdout and stderr to
/dev/null (just as it was prior to commit d28aa5a). Fixes #316

container/dangerzone.py

@@ -48,7 +48,13 @@ def run_command(
  """
  if stdout_callback is None and stderr_callback is None:
  try:
- subprocess.run(args, timeout=timeout, check=True)
+ subprocess.run(
+ args,
+ timeout=timeout,
+ stdout=subprocess.DEVNULL,
+ stderr=subprocess.DEVNULL,
+ check=True
+ )
  except subprocess.CalledProcessError as e:
  raise RuntimeError(error_message) from e
  except subprocess.TimeoutExpired as e: