add native Dockerfile for relay-gateway (replacing nixpacks)

Dockerfile

@@ -0,0 +1,8 @@
+FROM python:3.12-slim-bookworm
+WORKDIR /app
+RUN apt-get update && apt-get install -y --no-install-recommends build-essential && rm -rf /var/lib/apt/lists/*
+COPY requirements.txt ./
+RUN pip install --no-cache-dir -r requirements.txt
+COPY . .
+EXPOSE 18081
+CMD ["python", "-m", "relay_gateway.gateway"]