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.

Inizia da docker-compose.yml e ispeziona i servizi wp e init esistenti, quindi esamina la configurazione proposta di docker-demo/uploads.ini. Monta il file in entrambi i servizi con le due direttive PHP specificate e verifica che la demo Docker consenta upload fino a 8MB senza errori di upload PHP.

Scritto dal modello di indicizzazione a partire dal testo della issue.