Fix Dockerfile: replace invalid COPY with RUN mkdir for data dir

Dockerfile

@@ -10,7 +10,7 @@ RUN pip install --no-cache-dir -r requirements.txt
 # Copy project files
 COPY backend/ ./backend/
 COPY frontend/ ./frontend/
-COPY data/ ./data/ 2>/dev/null || mkdir -p ./data
+RUN mkdir -p ./data
 
 # HF Spaces runs as user with uid 1000
 RUN useradd -m -u 1000 user