chore: add containerised dev environment

[lgnap]

Run the harness, tests and build without installing Node 24 / npm 11 on the host. The image pins the versions from package.json "engines", so contributors get the same toolchain regardless of what their distro ships.

• Dockerfile: node:24-slim, deps installed in their own layer. npm ci uses --ignore-scripts because the root "prepare": "tsc" hook cannot run before the sources are copied, and is unnecessary: tsx executes TypeScript directly.
• docker-entrypoint.sh: creates an empty .env when absent (every npm script runs tsx --env-file=.env, which aborts on a missing file) and restores node_modules on the first run with a bind mount.
• docker-compose.yml: bind-mounts the tree with :z for SELinux, keeps node_modules in a named volume so it never lands in the host tree.
• Makefile: self-documenting targets (make park PARK=efteling, make test, make web...). Defaults to podman-compose; override with make COMPOSE="docker compose" dev on a Docker host.
• .dockerignore: excludes caches, secrets and build output from the context.

Verified in-container: make compile clean, make test 1536 tests passing.