I can reproduce this locally, without docker, with:

XDEBUG_CONFIG="" php -n -dzend_extension=xdebug -dxdebug.mode=debug -dxdebug.idekey="asdf" -r "var_dump(xdebug_is_debugger_Active());"

This is a duplicate of 0002070, which I have investigated and deemed as "working as expected": https://bugs.xdebug.org/2070#c6238

Thanks for your answer. I appreciate the update of the docs!

We will try to use XDEBUG_CONFIG furthermore, because it's a nice and easy way to configure docker images from outside without changing the actual image.

I think there is a different (better?) way of doing that. Instead of using XDEBUG_CONFIG, you can make a XDEBUG_CLIENT_HOST environment variable in your docker-compose.yml file:

services:
  php:
    build:
      dockerfile: ${PWD}/.devcontainer/Dockerfile
    image: php-fpm
    environment:
      XDEBUG_MODE: ${XDEBUG_MODE}
      XDEBUG_CLIENT_HOST: ${XDEBUG_CLIENT_HOST}

And then in your php.ini/xdebug.ini, use that created environment variable as well. This is described at https://www.php.net/manual/en/configuration.file.php#example-27:

[xdebug]
xdebug.client_host=${XDEBUG_CLIENT_HOST}

That workaround is more or less exactly what I switched to following my last ticket. Can confirm it works.