Docker Compose file is invalid

Question

I am learning Docker. My docker-compose.yml file is like below.

version: '3.8'

services:
    php56:
        build:
        context: .
        dockerfile: Dockerfile.56
        ports:
        - 88:80
        volumes:
        - ./php56:/var/www/html/

    db:
        image: mysql
        command: --default-authentication-plugin=mysql_native_password
        restart: always
        volumes:
        - ./volumes/mysql:/var/lib/mysql
        environment:
        MYSQL_ROOT_PASSWORD: password #example of a password

    phpmyadmin:
        image: phpmyadmin
        restart: always
        ports:
        - 8080:80

My Docker.56 file is like below

FROM php:5.6-apache
RUN docker-php-ext-install mysqli
RUN mv "$PHP_INI_DIR/php.ini-production" "$PHP_INI_DIR/php.ini"

I am getting error like below

ERROR: The Compose file './docker-compose.yml' is invalid because:
Unsupported config option for services.db: 'MYSQL_ROOT_PASSWORD'
Unsupported config option for services.php56: 'context'
services.php56.build contains an invalid type, it should be a string, or an object
services.db.environment contains an invalid type, it should be an object, or an array