fix(railpack): fail fast when buildx is unavailable

Require Docker buildx before Railpack builds, normalize environment
variable keys before validation, and align private deploy key API docs with
the supported dockerfile build pack.

app/Http/Controllers/Api/ApplicationsController.php

@@ -650,7 +650,7 @@ class ApplicationsController extends Controller
                             'environment_name' => ['type' => 'string', 'description' => 'The environment name. You need to provide at least one of environment_name or environment_uuid.'],
                             'environment_uuid' => ['type' => 'string', 'description' => 'The environment UUID. You need to provide at least one of environment_name or environment_uuid.'],
                             'dockerfile' => ['type' => 'string', 'description' => 'The Dockerfile content.'],
-                            'build_pack' => ['type' => 'string', 'enum' => ['nixpacks', 'railpack', 'static', 'dockerfile', 'dockercompose'], 'description' => 'The build pack type.'],
+                            'build_pack' => ['type' => 'string', 'enum' => ['dockerfile'], 'description' => 'The build pack type.'],
                             'ports_exposes' => ['type' => 'string', 'description' => 'The ports to expose.'],
                             'destination_uuid' => ['type' => 'string', 'description' => 'The destination UUID.'],
                             'name' => ['type' => 'string', 'description' => 'The application name.'],