If your setup isn't working, and you can not figure out the reason why, running
docker compose down -v will remove the relevant containers and volumes,
allowing you to run docker compose up again from a clean slate.
You can run ./bootstrap.sh again to check the setup and detected paths for your source code destination
In extreme cases, clean everything: docker system prune --all
If you start your stable containers (not the master) and it wants to install Nextcloud even if it is not the first start, you may have removed the configuration with the last docker compose down command. Try to use docker compose stop instead or give the stable setup named values yourself.
Logs
You can use docker compose logs -f to follow the logs of all containers
You can use docker compose logs -f nextcloud to follow the logs of the Nextcloud container