Harden production launch defaults

Dockerfile

@@ -4,6 +4,10 @@ WORKDIR /app
 COPY pyproject.toml README.md /app/
 COPY router_dyndns /app/router_dyndns
 RUN pip install --no-cache-dir .
+RUN adduser --system --group --home /app routerpulse \
+    && mkdir -p /data \
+    && chown -R routerpulse:routerpulse /app /data
+USER routerpulse
 
 EXPOSE 8080
 CMD ["router-dyndns", "serve", "--host", "0.0.0.0", "--port", "8080"]