fix: create /app/data dir with correct permissions for non-root user

Dockerfile

@@ -39,8 +39,8 @@ RUN pip install --no-cache-dir --upgrade pip \
 # Copy backend python packages
 COPY --chown=user backend/ ./
 
-# Establish web serving static destination directory and model directory
-RUN mkdir -p /app/static /app/models && chown -R user:user /app/static /app/models
+# Establish web serving static destination directory, model directory, and data directory
+RUN mkdir -p /app/static /app/models /app/data && chown -R user:user /app/static /app/models /app/data
 COPY --chown=user --from=builder /app/frontend/out /app/static
 
 # Enforce secure runner execution ownership context