Adds socket to config/database.php for consistency

For the mysql driver only (as this only applies to mysql) add the "socket" configuration parameter and corresponding environment variable.

config/database.php

@@ -46,6 +46,7 @@ return [
             'database' => env('DB_DATABASE', 'forge'),
             'username' => env('DB_USERNAME', 'forge'),
             'password' => env('DB_PASSWORD', ''),
+            'unix_socket' => env('DB_SOCKET', ''),
             'charset' => 'utf8mb4',
             'collation' => 'utf8mb4_unicode_ci',
             'prefix' => '',