Chore: Bump Docker demo environment PHP file upload size limit to 8MB

### Summary
In the local Docker demo environment, media uploads (images, WXR import files, themes...) larger than PHP's 2MB default were failing. Bumping `upload_max_filesize` and `post_max_size` to 8MB permits testing realistic media assets without modifying memory limits.

### Proposed Solution
1. Add `docker-demo/uploads.ini` configuring PHP upload directives:
- `upload_max_filesize = 8M`
- `post_max_size = 8M`
2. Mount `uploads.ini` into both the `wp` (Apache/PHP) and `init` (CLI) container services in `docker-compose.yml`

### Acceptance Criteria
- [ ] Docker demo permits file uploads up to the configured limit without PHP upload errors.

Comienza con docker-compose.yml e inspecciona los servicios wp e init existentes; después, revisa la configuración propuesta de docker-demo/uploads.ini. Monta el archivo en ambos servicios con las dos directivas PHP especificadas y verifica que la demo de Docker permita cargas de hasta 8MB sin errores de carga de PHP.

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