feat: add full Zonemaster stack with Docker and Spanish UI

- Clone all 5 Zonemaster component repos (LDNS, Engine, CLI, Backend, GUI)
- Dockerfile.backend: 8-stage multi-stage build LDNS→Engine→CLI→Backend
- Dockerfile.gui: Astro static build served via nginx
- docker-compose.yml: backend (internal) + frontend (port 5353)
- nginx.conf: root redirects to /es/, /api/ proxied to backend
- zonemaster-gui/config.ts: defaultLanguage set to 'es' (Spanish)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zonemaster/docs/public/installation/docker.md

@@ -0,0 +1,17 @@
+# Docker
+
+The following components are available on [Docker Hub], and can conveniently be
+downloaded and run without any installation. Through [Docker], Zonemaster can
+be run on Linux, MacOS and Windows.
+
+- Zonemaster-CLI: see [Using Zonemaster-CLI Docker] for how to run Zonemaster-CLI on [Docker].
+- Zonemaster-Backend: see [Using Zonemaster-Backend Docker] for how to run Zonemaster-Backend
+and Zonemaster-CLI on [Docker].
+
+To build your own Docker image, see the [Docker Image Creation] documentation.
+
+[Docker]:                              https://www.docker.com/get-started/
+[Docker Hub]:                          https://hub.docker.com/u/zonemaster
+[Docker Image Creation]:               https://github.com/zonemaster/zonemaster/blob/master/docs/internal/maintenance/ReleaseProcess-create-docker-image.md
+[Using Zonemaster-Backend Docker]:     ../using/backend/Using-Zonemaster-Backend-Docker.md
+[Using Zonemaster-CLI Docker]:         ../using/cli.md