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/using/backend/Using-Zonemaster-Backend-Docker.md

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+# Using Zonemaster-Backend Docker container
+
+## Table of contents
+
+* [Introduction](#introduction)
+* [Invoking `zmtest` using Docker](#invoking-zmtest-using-docker)
+* [Invoking `zmb` using Docker](#invoking-zmb-using-docker)
+* [Invoking the command line tool using Docker]
+* [IPv6 support](#ipv6-support)
+
+
+## Introduction
+
+This Docker image lets you run a working instance of Zonemaster-Backend.
+It also contains a working version of zonemaster-cli which can be used
+as a substitute for [zonemaster-cli],
+how to use this version of cli is described at
+[the end of this document][Invoking the command line tool using Docker].
+
+The container is configured to use an SQLite database and run 
+all needed processes (rpcapi and testagent).
+This image is designed to be used by one single user and
+is therefore not suitable for production.
+
+There is a limitation in Docker regarding IPv6.
+Unless IPv6 has been enabled in the Docker daemon, there is no support for IPv6.
+To avoid unnecessary errors,
+use the corresponding option below if IPv6 support is not available.
+See also the "IPv6 Support" section.
+
+- zonemaster-cli: `--no-ipv6`
+- zmtest: `--noipv6`
+- zmb: `--ipv6 false`
+
+
+## Invoking `zmtest` using Docker
+
+The most basic use of the `zonemaster-backend` command is to just test a domain, e.g.
+"zonemaster.net". To do so you first need to start `zonemaster/backend`
+container and expose the port 5000
+to be able to access the JSON RPC API.
+
+```
+docker run --rm -p 5000:5000 --name zm -d zonemaster/backend full
+```
+You can stop the Docker container with `docker stop zm`.
+
+Once the zonemaster/backend container is started you can interact with it on 
+`localhost:5000` using JSON RPCAPI.
+
+You can use `zmtest` embedded inside the backend image by using this command:
+```
+docker run -ti --rm --net host zonemaster/backend zmtest zonemaster.net
+```
+Run 
+```sh
+docker run -ti --rm --net host zonemaster/backend zmtest zonemaster.net
+```
+to get usage for `zmtest`.
+
+
+## Invoking `zmb` using Docker
+
+You can also use the `zmb` command to interact with the `zonemaster/backend` container.
+
+```
+docker run -ti --rm --net host zonemaster/backend zmb start_domain_test --domain zonemaster.net
+docker run -ti --rm --net host zonemaster/backend zmb get_test_results --test-id ed38765834e45b6e --lang en
+```
+
+When piping the output of `docker run` directly into `jq`,
+you might encounter display issues due to the way Docker handles output.
+These issues are caused by carriage return characters (^M) in the output.
+To filter out these characters,
+you should pipe `docker run`’s output into `tr -d '^M'` before piping it into `jq`.
+
+```
+docker run -ti --rm --net host zonemaster/backend zmb get_test_results (...) |tr -d '^M' | jq
+```
+Run
+```sh
+docker run -ti --rm --net host zonemaster/backend zmb
+```
+or 
+```
+docker run -ti --rm --net host zonemaster/backend zmb man
+```
+to get usage and man page, respectively, for `zmb`.
+
+## Invoking the command line tool using Docker
+
+`zonemaster/backend` also contains the CLI tool shipped in 
+the [zonemaster-cli] container image.
+You can invoke zonemaster-cli in the Zonemaster-Backend Docker image 
+by running the following command:
+
+```
+docker run -ti --rm zonemaster/backend:local cli zonemaster.net
+```
+
+
+## IPv6 support
+
+On a Linux system IPv6 support can be enabled by creating or updating
+`/etc/docker/daemon.json`. This is a minimal file that enables IPv6 support:
+
+```json
+{
+    "ipv6": true,
+    "fixed-cidr-v6": "2001:db8:1::/64"
+}
+```
+
+Restart the docker daemon:
+```sh
+sudo systemctl restart docker
+```
+
+Also see the official Docker documentation "[Enable IPv6 support]".
+
+
+[Docker Image Creation]:                         https://github.com/zonemaster/zonemaster/blob/master/docs/internal/maintenance/ReleaseProcess-create-docker-image.md
+[Enable IPv6 support]:                           https://docs.docker.com/config/daemon/ipv6/
+[Get started]:                                   https://www.docker.com/get-started/
+[IPv6 support]:                                  #ipv6-support
+[Invoking the command line tool using Docker]:   #invoking-the-command-line-tool-using-docker
+[Zonemaster-cli]:                                ../cli.md