This commit fixes CORS errors when running the app in Docker by introducing the `APP_URL` environment variable. A CORS policy is set up for the backend to only allow origin from the `APP_URL`.
Key changes include:
- New `APP_URL` and `ORIGIN` environment variables have been added to properly configure CORS and the SvelteKit adapter, making the application's public URL easily configurable.
- Dockerfiles are updated to copy the entrypoint script, Drizzle config, and migration files into the final image.
- Documentation and example files (`.env.example`, `docker-compose.yml`) have been updated to reflect these changes.
