added docker-compose.yml with composer, test, psalm and format

[dam-bal]

This will make local development easier

docker-compose run --rm composer install to install composer dependencies
docker-compose run --rm test to run tests
docker-compose run --rm psalm to run psalm
docker-compose run --rm format to format with php-cs-fixer

I had to add platform.php to composer.json as it will be using official composer image which might use different PHP version and there might be issues around dependencies.

[AlexVanderbist]

Hi @dam-bal, thanks for your work on this package! We generally avoid including details about the development environment in our packages. This allows every contributor to decide how to want to run tests for themselves, and keeps us from having to maintain multiple docker-compose/setup scripts for every every package.

As an alternative, I'd propose keeping your own docker-compose.yml in the repo files but working with a global .gitignore to avoid accidentally committing it for this specific repo.

Thanks!