You have to provide the right socket path in the Nextcloud configuration file (config.php) too. If you want to use a socket connection it should look similar to this:
'dbname' => 'nextcloud',
'dbtableprefix' => 'oc_',
'dbtype' => 'mysql',
'dbhost' => 'localhost:/<path-to-socket>/mysql.sock',
'dbuser' => '<dbuser>',
'dbpassword' => '<password>',