NextCloud Flow within Docker container

Hi everyone,
I am running NextCloud within docker (based on “with-nginx-proxy/mariadb/fpm”) which works very well. But I can not get Flow working with external scripts.

I want to run a script when a defined file is opened. I have installed the “external script” app and configured a flow with:
“File accessed” and file name is “nextcloud_flow_test.txt” run script at “/var/www/html/config/test.sh”.

But the script is not executed when accessing the file from my NextCloud Webinterface.

The script itself is accessable from the docker container. I started it via an interactive shell from the container (docker exec -it nextcloud bash). Currently the script just inserts the current date to a text file (date >> test.txt) which works fine when executed manually.

Is there anything I am missing? Should the Flow be executed right away or is this done via the background job (in my case every minute via cron, but I waited longer than than multiple times)?