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.