dstack-ingress: add WebSocket proxy support

The `kvin/dstack-ingress:wildcard` image generates its nginx config via `setup_nginx_conf()` in the entrypoint, but the `location /` block is missing WebSocket upgrade headers. Without these, any WebSocket service behind dstack-ingress gets HTTP 200 instead of 101 Upgrade.

**Suggested fix**: add these three lines to the `location /` block in `setup_nginx_conf()`:

```nginx
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
```

This is safe for mixed HTTP + WebSocket traffic. `$http_upgrade` is empty for regular HTTP requests, and upstreams ignore `Connection: upgrade` when there's no valid `Upgrade` value. `proxy_http_version 1.1` is strictly better than the default 1.0 (also enables keepalive). This is the standard nginx pattern recommended in the [nginx WebSocket docs](https://nginx.org/en/docs/http/websocket.html).

**Current workaround**: override `command` in docker-compose to patch the generated config before starting nginx:

```yaml
command:
- bash
- -c
- |
sed -i '/proxy_pass/a\ proxy_http_version 1.1;\n proxy_set_header Upgrade $$http_upgrade;\n proxy_set_header Connection "upgrade";' /etc/nginx/conf.d/default.conf
exec nginx -g "daemon off;"
```