The error you're receiving refers to Symfony runtime environment variables (see here). Docker compose environment variables are retrieved from the .env file that resides within the build context (AKA the directory where you want docker-compose to run from) of the docker-compose.yml file. What you really want to do is make sure that the environment variables you set in your Symfony config/parameters and your docker-compose.yml file match, especially the db host.

You should consider breaking your image up into PHP-FPM and Nginx images to have better potential scalability and a separation of concerns according to the best practices outlined by Docker here. All of these technologies have regularly-maintained images available on Docker Hub.

Here's my docker-compose.yml that creates containers for PHP-FPM, MySQL, and Nginx in a multicontainer environment with the latest stable packages available as of today.

version: '3'

services:
    db:
        image: mysql
        volumes:
            - ./.data/db:/var/lib/mysql
        environment:
            MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD}
            MYSQL_DATABASE: ${MYSQL_DATABASE}
            MYSQL_USER: ${MYSQL_USER}
            MYSQL_PASSWORD: ${MYSQL_PASSWORD}
    php:
        build: php7.1-fpm
        depends_on:
            - db
        ports:
            - "9000:9000"
        volumes:
            - symfony-bind-mount:/var/www/symfony
            - symfony-logs-bind-mount:/var/www/symfony/app/logs
    nginx:
        build: nginx
        depends_on:
            - php
        ports:
            - "8025:8025"
            - "80:80"
        volumes:
            - symfony-bind-mount:/var/www/symfony
            - nginx-logs-bind-mount:/var/log/nginx

volumes:
    symfony-bind-mount:
        driver: local
        driver_opts:
            o: bind,rw
            type: none
            device: ${SYMFONY_APP_PATH}
    nginx-logs-bind-mount:
        driver: local
        driver_opts:
            o: bind,rw
            type: none
            device: ${DOCKER_SYMFONY_PATH}/logs/nginx
    symfony-logs-bind-mount:
        driver: local
        driver_opts:
            o: bind,rw
            type: none
            device: ${DOCKER_SYMFONY_PATH}/logs/symfony

Here's my .env file:

# Symfony application's path
SYMFONY_APP_PATH=/absolute/path/to/symfony/project

# Path to local docker-symfony repository
DOCKER_SYMFONY_PATH=/absolute/path/to/sibling/docker/directory

# MySQL
MYSQL_ROOT_PASSWORD=root
MYSQL_DATABASE=mydb
MYSQL_USER=
MYSQL_PASSWORD=password

You can checkout my fork of maxpou's docker-symfony repository here. I've updated the docker-compose.yml and the bind mounts to be compatible with version 3 of the Compose file format.