Where is dockerfile located in default laravel sail project

Building a laravel project using laravel sail documentation. As mentioned in the docker-compose.yml file it mentions dockerfile: Dockerfile. But it is not located in the root folder, so where is it?

# For more information: https://laravel.com/docs/sail
version: '3'
services:
    laravel.test:
        build:
            context: ./vendor/laravel/sail/runtimes/8.1
            **dockerfile: Dockerfile**
            args:
                WWWGROUP: '${WWWGROUP}'
        image: sail-8.1/app
        extra_hosts:
            - 'host.docker.internal:host-gateway'
        ports:
            - '${APP_PORT:-80}:80'
        environment:
            WWWUSER: '${WWWUSER}'
            LARAVEL_SAIL: 1
            XDEBUG_MODE: '${SAIL_XDEBUG_MODE:-off}'
            XDEBUG_CONFIG: '${SAIL_XDEBUG_CONFIG:-client_host=host.docker.internal}'
        volumes:
            - '.:/var/www/html'
        networks:
            - sail
        depends_on:
            - mysql
            - redis
            - meilisearch
            - selenium