Add Docker development and test stack

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Runs Chamilo 2.0 locally with only Docker installed — no host PHP, MySQL, Node.js or Apache. Covers the app plus all four test tiers (PHPUnit, Behat, PHPStan/Psalm, ECS).

docker/setup.sh build, start, install the portal, prepare the test DB
docker/test.sh run any test tier
docker/set-port.sh change the published port consistently

Services: PHP 8.2 + Apache, MariaDB 10.11, Redis, Mailpit, Selenium. All images are multi-arch, so this works on both Intel and Apple Silicon.

Notes on the less obvious choices, all documented in docker/README.md:

• Apache listens on 8380 inside the container and is published on the same host port. Chamilo persists root_web at install time, so the container port and the browser port must match or every redirect breaks.
• Selenium is pinned to 4.1.4. behat/mink-selenium2-driver 1.7 speaks only the legacy JSON Wire protocol; newer grids reject it outright.
• Behat uses tests/behat/behat.docker.yml rather than BEHAT_PARAMS, because Behat 3.29 lets the file config win over the environment variable.
• .env is never created empty. Symfony falls back to .env.dist only when .env is absent, and that fallback supplies the pre-install defaults.
• .env.local / .env.test.local stay gitignored but are generated from templates in docker/env/, since the stack cannot start without them.

Docs: SETUP.md is the step-by-step guide; docker/README.md explains the design.