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/README.md

@@ -0,0 +1,87 @@
+# Build Environment Preparation
+
+## Overview
+
+The build environment is used for several purposes, but the purpose
+is not for the actual installation of Zonemaster. For the installation
+the normal installation instructions for users should be used. In most
+cases, if you create a build environment you usually do not install
+Zonemaster on that.
+
+* When testing the installation instructions for users you should avoid
+  to do that from a build environment.
+* For the use cases below, your should in most cases make sure that
+  you use the develop branch.
+* You should probably read this file
+  [from the develop branch][BuildEnvironmentPreparation], not master
+  branch to get the latest changes.
+
+## Use cases
+
+1. Creating Perl tarballs for testing 1).
+2. Creating Perl tarballs for uploading to CPAN as part of a Zonemaster
+   release 1).
+3. Creating Zonemaster-GUI zip distribution for testing.
+4. Creating Zonemaster-GUI zip distribution for release.
+5. Translation work (PO file updates).
+6. Updating documents in Zonemaster/Zonemaster by scripts in
+   [utils README] as part of release.
+7. Check for broken, internal links in mdBook as part of release.
+8. Development work.
+
+There could be more use cases.
+
+1\) Zonemaster-LDNS, Zonemaster-Engine, Zonemaster-CLI and Zonemaster-Backend.
+
+
+### Building Perl CPAN packages
+
+To create a Perl tarball (use cases 1 and 2) you need a specific environment.
+Once you have set up your build environment follow the building instructions as
+described in [Create Test Distribution] document of the release process.
+
+Build environments:
+* [Debian build environment]
+* [FreeBSD build environment]
+* [Ubuntu build environment]
+* Rocky-Linux environment (TBD)
+
+### GUI zip distribution
+
+For use cases 3 and 4, install [Ubuntu Node.js environment].
+
+### Translation work
+
+For use case 5 (which applies for Zonemaster-Engine and
+Zonemaster-CLI) install one of the environments listed for
+use case 1 and 2 and then follow the Zonemaster-Engine
+[instructions for translators].
+
+### Updating documents
+
+For use case 6, install one of the environments listed
+for use cases 1 and 2, and then follow the
+[installation instructions] for Zonemaster-Engine. Make sure
+that you install from develop branch.
+
+### Check for broken, internal links in mdBook
+
+For use case 7, install according to the build environment
+instructions (see above).
+
+### Development work
+
+For use case 8, install an environment as for use cases
+1 or 3. Additional installation might be needed.
+
+
+<!-- Zonemaster links point on purpose on the develop branch. -->
+[BuildEnvironmentPreparation]:        https://github.com/zonemaster/zonemaster/blob/develop/docs/internal/distrib-testing/README.md
+[Debian build environment]:           Debian-build-environment.md
+[FreeBSD build environment]:          FreeBSD-build-environment.md
+[Ubuntu build environment]:           Ubuntu-build-environment.md
+[Ubuntu Node.js environment]:         Ubuntu-Node.js-build-environment.md
+[Create Test Distribution]:           ../maintenance/ReleaseProcess-create-test-distribution.md
+[Installation instructions]:          https://github.com/zonemaster/zonemaster/blob/develop/docs/public/installation/zonemaster-engine.md
+[instructions for translators]:       https://github.com/zonemaster/zonemaster/blob/develop/docs/internal/maintenance/Instructions-for-translators.md
+[utils README]:                       ../../../utils/README.md