In docker-compose.yml the alternative XDEBUG_CONFIG is missleading.
Multiple configs are separated by space.
So with the proposed
XDEBUG_CONFIG: remote_host=host.docker.internal, remote_port=9001
XDebug tries to make a connection to http://host.docker.internal,:9001 and fails.
Working config is:
XDEBUG_CONFIG: remote_host=host.docker.internal remote_port=9001
By the way: Validate Debugger Configuration on Web Server shows all green check marks even with wrong config.