Use ENV variables for services definitions

Hello!

I very like pipelines and I want to thank the team for that useful service.

I have a suggestion to use ENV variables for definitions section.
I'm developing laravel project and I think that using ENV vars for services credentials would be very convenient.

#!yml

definitions:
  services:
    mysql:
      image: mysql:8
      environment:
        MYSQL_DATABASE: ${DB_DATABASE}
        MYSQL_USER: ${DB_USERNAME}
        MYSQL_PASSWORD: ${DB_PASSWORD}
        MYSQL_RANDOM_ROOT_PASSWORD: yes

I know that for test env it is not critical, but I think that specifying services credentials in one place would be more convenient.

Thanks!