andrewjs18 here's a stripped version from my previous hosting stack: linkrobins/flarum-docker
But to answer your question, I didn't look at the docker image you shared at all and have no experience with it. They'll need to make tweaks on however way they manage the config install to be mapped to mariadb specifically. Mine is mariadb native with the config properly set based on Flarum docs.
# Write a fresh Flarum config.php from env (used by the restore path, which
# imports the DB dump instead of running `flarum install`).
write_config_php() {
cat > "$CONFIG_FILE" <<PHP
<?php return [
'debug' => false,
'database' => [
'driver' => 'mariadb',
'host' => '${DB_HOST}',
'port' => 3306,
'database' => '${DB_NAME}',
'username' => '${DB_USER}',
'password' => '${DB_PASS}',
'charset' => 'utf8mb4',
'collation' => 'utf8mb4_unicode_ci',
'prefix' => '',
'prefix_indexes' => true,
'strict' => false,
'engine' => null,
],
'url' => '${APP_URL}',
'paths' => ['api' => 'api', 'admin' => 'admin'],
];
PHP
chown www-data:www-data "$CONFIG_FILE"
}
