Dockerfile-可能的命令行参数?

${APP_NAME} . 它是什么?如果这是bash脚本,我会假设它是某种变量,但它在任何地方都没有赋值。它是命令行参数吗?如果是的话,我想打电话的时候怎么传过来 docker-compose 用它?

version: '2'
services:
    nginx:
        container_name: ${APP_NAME}_nginx
        hostname: nginx
        build:
            context: ./containers/nginx
            dockerfile: Dockerfile
        ports:
            - "80:80"
            - "443:443"
        volumes:
            - .:/app
        links:
            - phpfpm
        networks:
            - backend

    phpfpm:
        container_name: ${APP_NAME}_phpfpm
        hostname: phpfpm
        expose:
            - "9000"
        build:
            context: ./containers/php-fpm
            dockerfile: Dockerfile
        working_dir: /app
        volumes:
            - .:/app
        links:
            - mysql
        networks:
            - backend

    mysql:
        container_name: ${APP_NAME}_mysql
        hostname: mysql
        build:
            context: ./containers/mysql
            dockerfile: Dockerfile
        volumes:
            - ./storage/mysql:/var/lib/mysql
            - ${MYSQL_ENTRYPOINT_INITDB}:/docker-entrypoint-initdb.d
        environment:
            - MYSQL_DATABASE=${DB_DATABASE}
            - MYSQL_ROOT_PASSWORD=${DB_PASSWORD}
        ports:
            - "33061:3306"
        expose:
            - "3306"
        networks:
          - backend

networks:
    backend:
        driver: "bridge"