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.

Start with docker-compose.yml and inspect the existing wp and init services, then review the proposed docker-demo/uploads.ini configuration. Mount the file into both services with the two specified PHP directives, and verify that the Docker demo permits uploads up to 8MB without PHP upload errors.

Written by the indexing model from the issue text.