fix(railpack): pin frontend image version via config constant

Remove RAILPACK_FRONTEND_IMAGE env var from helper Dockerfile and resolve
the image ref at runtime using a new `railpack_version` constant in config.
Eliminates Docker build-time env interpolation for BUILDKIT_SYNTAX arg.

config/constants.php

@@ -5,6 +5,7 @@ return [
         'version' => '4.1.0',
         'helper_version' => '1.0.13',
         'realtime_version' => '1.0.14',
+        'railpack_version' => '0.22.0',
         'self_hosted' => env('SELF_HOSTED', true),
         'autoupdate' => env('AUTOUPDATE'),
         'base_config_path' => env('BASE_CONFIG_PATH', '/data/coolify'),