Hi @imam_ghazali , please check the following:-
- Are you running docker dev container reference:- frappe_docker/docs/development.md at main · frappe/frappe_docker · GitHub and not on production based containers.
Production dockers are not supported to create custom apps and so if you want to develop the app ideally you should run dev dockers.
And once you complete building apps, you may can deploy referencing frappe_docker/docs/custom-apps.md at main · frappe/frappe_docker · GitHub
IMHO, it is better to develop apps not using containers if you are novice to bash commands. You may refer [Guide] How to install ERPNext v15 on Linux Ubuntu (step-by-step instructions) to install locally without docker.
But looking at longer picture its good to explore docker as it has a variety of advantages for development.
Shubham