Use config.json.example for Docker build

backend/Dockerfile

@@ -83,8 +83,8 @@ RUN cd build && \
              -DCMAKE_BUILD_WITH_INSTALL_RPATH=TRUE && \
     cmake --build . -j$(nproc)
 
-# Copy config
-COPY config.json .
+# Copy config template (real config mounted at runtime via docker-compose)
+COPY config.json.example config.json
 
 # Create uploads directory with proper permissions
 # Using nobody user's UID/GID (65534) for consistency with nginx