Add GPLv3 license headers to source files

Added GNU General Public License v3 headers to protocol and utility modules for Xget, clarifying copyright and licensing information. This ensures compliance and transparency regarding the project's open source status.

src/protocols/docker.js

@@ -1,3 +1,21 @@
+/**
+ * Xget - High-performance acceleration engine for developer resources
+ * Copyright (C) 2025 Xi Xu
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <https://www.gnu.org/licenses/>.
+ */
+
 /**
  * Docker/OCI Registry protocol handler for Xget
  */