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/Ubuntu-build-environment.md

@@ -0,0 +1,118 @@
+# Ubuntu Build Environment
+
+## Table of contents
+
+* [Introduction](#introduction)
+* [Preparation](#preparation)
+* [Installation for package building](#installation-for-package-building)
+* [Install Docker](#install-docker)
+* [Translation work](#translation-work)
+* [Generate documents in Zonemaster/Zonemaster](#generate-documents-in-zonemasterzonemaster)
+* [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. You should normally use the version of these instructions found in the
+develop branch.
+
+This instruction is for creating it on Ubuntu. See other files for other OSs.
+
+This description targets to create a system that can be used for the following
+needs for testing and releasing Zonemaster:
+
+1. Git work on the Zonemaster repositories.
+2. Creating distributing packages for testing or uploading to CPAN.
+3. Creating Docker imaging for testing or uploading to Docker Hub.
+4. Updating PO files (translation).
+5. Generating the files listed in the [utils README].
+
+
+## Preparation
+
+1. Make a clean installation of [Ubuntu] 22.04.
+
+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 libssl-dev libtest-exception-perl 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
+   ```
+
+## Install Docker
+
+This step is only needed if Docker images are to be built.
+
+The Docker version installed by the command below is the version found in the
+Ubuntu package repository. If a newer version is needed, follow the
+instructions on the [Install Docker Engine on Ubuntu] page instead.
+
+   ```sh
+   sudo apt-get install docker.io
+   ```
+
+Add yourself to the Docker group.
+
+   ```sh
+   sudo usermod -aG docker $LOGNAME
+   ```
+
+## 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
+  Ubuntu (usually use the version in develop branch).
+
+## Generate documents in Zonemaster/Zonemaster
+
+Only needed if the files listed in [utils README] are to be generated (updated).
+
+* Follow the [Installation instructions] for Zonemaster-Engine for Ubuntu.
+  * Only install the dependencies from binary packages and CPAN (if any).
+  * Do not install neither Zonemaster::LDNS nor Zonemaster::Engine at this stage.
+
+## 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
+   ```
+
+
+
+[Install Docker Engine on Ubuntu]:         https://docs.docker.com/engine/install/ubuntu/
+[Installation instructions]:               ../../public/installation/zonemaster-engine.md
+[Instructions for translators]:            ../maintenance/Instructions-for-translators.md#software-preparation
+[Ubuntu]:                                  https://ubuntu.com/
+[Utils README]:                            ../../../utils/README.md