feat: update docker file

backend/Dockerfile

@@ -6,6 +6,8 @@ RUN apk add --no-cache tzdata && \
     echo "Asia/Shanghai" > /etc/timezone && \
     apk del tzdata
 
+RUN mkdir /app/downloads /app/dist
+
 COPY backend/build/app /app/app
 COPY backend/config.prod.toml /app/config.toml
 COPY frontend/admin/dist /app/dist/admin