feat(demo): add Scriptor 2.0 Docker demo stack

[bigin]

Adds a one-command Docker stack to try Scriptor 2.0 without installing PHP, Composer, or SQLite locally:

docker compose up -d --build

…brings Scriptor up on http://localhost:8080 with a single admin user (admin / scriptor) and one example page seeded on first start.

Stack shape (matches the iManager deployment guide):

• scriptor service — php:8.3-fpm-alpine with the iManager extensions (pdo_sqlite, mbstring, gd, dom, zip, opcache), production-shaped opcache (with timestamp validation on so demo restarts pick up code changes), Composer install of bigins/scriptor + bigins/imanager.
• web service — nginx:alpine fronting it; standard front-controller routing with /data/uploads-2.0/ served directly and the rest of data/ blocked at the URL layer.
• Both services mount a shared named volume scriptor-app at /var/www/scriptor; web mounts it read-only.
• docker compose down -v resets the demo to factory state.

Composer install in the image overrides composer.json's path-repo (../imanager) with a VCS repo pointing at the public iManager GitHub repo, so the demo build doesn't need a sibling iManager checkout.

Seed (docker/seed-demo.php) is idempotent. It runs on first start only — if a pages category already exists it exits cleanly. It creates:

• Pages category + 7 fields (slug, parent, pagetype, menu_title, content, template, images).
• Users category + 3 fields (role, email, password).
• Admin user — admin / scriptor, role=admin. Password is passed as plaintext to ItemRepository::save() so the PasswordFieldType does the bcrypt hashing.
• One Hello-world example page that explains what's been seeded.

Schema migrations are reapplied on every container start (cheap no-op when already current). The library's DefaultBootstrap would auto-migrate on first PDO resolve anyway; the explicit step in the entrypoint gives clearer log output and survives a future entrypoint that doesn't immediately open a connection.

Files:

• docker/Dockerfile (74 lines)
• docker/nginx.conf (75 lines)
• docker/entrypoint.sh (43 lines)
• docker/seed-demo.php (138 lines, idempotent seed)
• docker-compose.yml (45 lines)
• docs/demo.md (149 lines — quickstart + what-the-seed-
  creates + explorations + reset + when-
  NOT-to-use)
• README.md (+ a "Try it in Docker" subsection
  pointing at docs/demo.md)

This rounds out the Phase 16 deliverables on the Scriptor side; the library-side docs (API ref, cookbooks, deployment guide, migration guide) all landed earlier.