_ss_environment.php

<?php
/* Change this from 'dev' to 'live' for a production environment. */
define('SS_ENVIRONMENT_TYPE', 'dev');

/* This defines a default database user */
define('SS_DATABASE_SERVER', 'database');
define('SS_DATABASE_NAME', 'SS_mysite');
define('SS_DATABASE_USERNAME', 'root');
define('SS_DATABASE_PASSWORD', '');

/* Configure a default username and password to access the CMS on all sites in this environment. */
define('SS_DEFAULT_ADMIN_USERNAME', 'admin');
define('SS_DEFAULT_ADMIN_PASSWORD', 'password');

// This is used by sake to know which directory points to which URL
global $_FILE_TO_URL_MAPPING;
$_FILE_TO_URL_MAPPING['/var/www/html'] = 'http://localhost';

docker-compose.yml

version: '3'
services:
 web:
   image: brettt89/silverstripe-web
   working_dir: /var/www
   ports:
        - 8080:80
   volumes:
   - .:/var/www/html
   networks:
          - web
          - database

 database:
   image: mysql
   volumes:
    - db-data:/var/lib/mysql
   restart: always
   networks:
         - database
   environment:
    - MYSQL_ALLOW_EMPTY_PASSWORD=true

volumes:
 db-data:

networks:
  web: null
  database: null