How to access the hostname of mariadb in the nginx server

$CFG->dbtype    = 'mariadb';      // 'pgsql', 'mariadb', 'mysqli', 'auroramysql', 'sqlsrv' or 'oci'
$CFG->dblibrary = 'native';     // 'native' only at the moment
$CFG->dbhost    = getenv('MOODLE_DB_HOST') ?: 'localhost';  // eg 'localhost' or 'db.isp.com' or IP
$CFG->dbname    = 'moodle';     // database name, eg moodle
$CFG->dbuser    = 'mariadb';   // your database username

      envVariables:
        MOODLE_DB_TYPE: mariadb
        MOODLE_DB_HOST: mariadb
        MOODLE_DB_PORT: 3306
        MOODLE_DB_NAME: moodle
        MOODLE_DB_USER: mariadb