Changed Docker installation doc to MariaDB

[BielefeldJ]

Changed the installation documentation for docker to MariaDB.

Reason for this is, that the current documentation still uses "mysql_native_password" with is deprecated and will be removed. Instead of just removing this, I switched the docs to MariaDB to be more consistent with the example docker-compose.yml from the espocrm-docker repo.

I had to add the ESPOCRM_DATABASE_HOST=mariadb as env for the EspoCRM container, as the default hostname is mysql. Changing the default hostname in the espocrm-docker project would probably kill some installations (if they are using the default settings). So I would not recommend that.