docs: make Docker HTTP example portable to native Linux

Add --add-host=host.docker.internal:host-gateway so the container can
reach a host SearXNG on native Linux Docker, where host.docker.internal
does not resolve automatically (unlike Docker Desktop). Addresses Copilot
review feedback on PR #165.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: ihor-sokoliuk

README.md

@@ -238,11 +238,14 @@ Or with Docker (bind to all interfaces so the port is reachable from the host):
 
 ```bash
 docker run --rm -p 3000:3000 \
+  --add-host=host.docker.internal:host-gateway \
   -e MCP_HTTP_PORT=3000 -e MCP_HTTP_HOST=0.0.0.0 \
   -e SEARXNG_URL=http://host.docker.internal:8080 \
   isokoliuk/mcp-searxng:latest
 ```
 
+The `--add-host` mapping lets the container reach a SearXNG instance on the host via `host.docker.internal`; it resolves automatically on Docker Desktop but needs this flag on native Linux. Point `SEARXNG_URL` at your actual instance if it runs elsewhere.
+
 **Connect an HTTP-capable MCP client** to the `/mcp` endpoint by URL:
 
 ```json