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/internal/distrib-testing/Debian-build-environment.md

@@ -0,0 +1,75 @@
+# Debian Build Environment
+
+## Table of contents
+
+* [Introduction](#introduction)
+* [Preparation](#preparation)
+* [Installation for package building](#installation-for-package-building)
+* [Translation work](#translation-work)
+* [Installation for mdBook](#installation-for-mdbook)
+
+## Introduction
+
+These are instructions for creating a build environment for Zonemaster
+components based on Perl. This is not meant as instructions for installing
+Zonemaster itself. 
+
+This instruction is for creating it on Debian. See other files for other OSs.
+
+## Preparation
+
+1. Make a clean installation of latest version of Debian.
+
+2. Update the package database.
+
+   ```sh
+   sudo apt-get update
+   ```
+
+## Installation for package building
+
+1. Install dependencies and tools:
+
+   ```sh
+   sudo apt-get install git cpanminus gettext autoconf automake build-essential libdevel-checklib-perl libextutils-pkgconfig-perl libmime-base32-perl libmodule-install-xsutil-perl libtest-differences-perl libtest-exception-perl libssl-dev libidn2-dev libtool
+   ```
+
+2. Clone 'develop' branch from all Zonemaster repositories except GUI:
+
+   ```sh
+   git clone -b develop https://github.com/zonemaster/zonemaster.git
+   for d in ldns engine cli backend; do git clone -b develop https://github.com/zonemaster/zonemaster-$d.git; done
+   ```
+
+## Translation work
+
+Install for translation (handling PO files), only needed if PO files are to be
+handled.
+
+* Follow "Software preparation" in [Instructions for translators] for
+  Debian (usually use the version in develop branch).
+
+
+## Installation for mdBook
+
+> Note that building with Cargo below can be time consuming.
+
+Needed for release process:
+
+   ```
+   sudo apt install rustc
+   ```
+   ```
+   cargo install mdbook-linkcheck
+   ```
+Needed to build the mdBook (not part of release process):
+
+   ```
+   sudo apt install rustc
+   ```
+   ```
+   cargo install mdbook mdbook-linkcheck
+   ```
+
+
+[Instructions for translators]:            ../maintenance/Instructions-for-translators.md#software-preparation