Add Docker support for headless server environments

## Problem

OpenAdapter requires a graphical desktop because Chromium runs headful (Cloudflare blocks headless). This makes deployment on servers difficult.

## Proposed fix

Create a Dockerfile that uses Xvfb (virtual framebuffer) to provide a fake display:

```dockerfile
RUN apt-get install -y xvfb
CMD ["xvfb-run", "node", "server.js"]
```