Package 'mysql-client' has no installation candidate in php-fpm image build using docker compose

Ref: https://stackoverflow.com/questions/57048428/e-package-mysql-client-has-no-installation-candidate-in-php-fpm-image-build-u

 php:7.3-fpm now use Debian 10 (Buster) as its base image and Buster ships with MariaDB, so just replace mysql-client with mariadb-client should fix it.