Support updating docker image for a server from the frontend

app/Http/Requests/Api/Client/Servers/Settings/SetDockerImageRequest.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace Pterodactyl\Http\Requests\Api\Client\Servers\Settings;
+
+use Webmozart\Assert\Assert;
+use Pterodactyl\Models\Server;
+use Illuminate\Validation\Rule;
+use Pterodactyl\Models\Permission;
+use Pterodactyl\Contracts\Http\ClientPermissionsRequest;
+use Pterodactyl\Http\Requests\Api\Client\ClientApiRequest;
+
+class SetDockerImageRequest extends ClientApiRequest implements ClientPermissionsRequest
+{
+    /**
+     * @return string
+     */
+    public function permission(): string
+    {
+        return Permission::ACTION_STARTUP_DOCKER_IMAGE;
+    }
+
+    /**
+     * @return array[]
+     */
+    public function rules(): array
+    {
+        /** @var \Pterodactyl\Models\Server $server */
+        $server = $this->route()->parameter('server');
+
+        Assert::isInstanceOf($server, Server::class);
+
+        return [
+            'docker_image' => ['required', 'string', Rule::in($server->egg->docker_images)],
+        ];
+    }
+}