feat(docker): migrate to Debian stable, add deployer CLI to builder stage

[t0kubetsu]

Summary

Closes #42

• Base image changed from Alpine to Debian stable (bookworm) — `node:24-bookworm-slim` for the builder, `nginx:stable-bookworm` for the runtime. Debian stable was chosen over Ubuntu LTS: it avoids Ubuntu-specific packaging quirks, has an equally long support lifecycle, and is the reference platform used in our existing Debian VM test environment.
• Node.js upgraded to 24 (latest stable) — compatible with the `>=22.12.0` engines constraint already defined in `package.json`.
• range42-deployer CLI installed in builder stage via a Python venv (`/opt/deployer-env`) — PEP 668 compliant (Debian bookworm, like Ubuntu 23.04+, blocks system-level `pip install`; a venv is the correct solution on both distros).
• Optional `DEPLOYER_CLI_VERSION` build arg for pinning the CLI to a specific release.
• `docker-compose.yml` now includes an explicit `healthcheck` block (mirrors the `HEALTHCHECK` instruction in the Dockerfile).
• `README.md` gains a Docker: Build & Push section with tag, push, and build-arg examples.

Test plan

• `docker compose up --build` succeeds and `docker compose ps` reports `healthy`
• `curl http://localhost:3000/health\` returns `ok`
• SPA loads at `http://localhost:3000\`
• `docker build --build-arg DEPLOYER_CLI_VERSION=1.2.3 .` completes without error (once a release exists)
• `docker run --rm range42-deployer --version` confirms the CLI is on `PATH`