如何让flutter应用程序在整个屏幕上包含一个伸展的背景图像，里面有一个返回句子的图像的圆形按钮？

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Stack(
          children: [
            // 背景图像
            Container(
              decoration: BoxDecoration(
                image: DecorationImage(
                  image: AssetImage('assets/background_image.jpg'),
                  fit: BoxFit.cover,
                ),
              ),
            ),
            // 返回句子的图像的圆形按钮
            Align(
              alignment: Alignment.bottomCenter,
              child: Padding(
                padding: EdgeInsets.only(bottom: 20.0),
                child: GestureDetector(
                  onTap: () {
                    // 点击按钮触发的操作
                    print('返回句子的图像的圆形按钮被点击了');
                  },
                  child: Container(
                    width: 80.0,
                    height: 80.0,
                    decoration: BoxDecoration(
                      shape: BoxShape.circle,
                      color: Colors.blue,
                    ),
                    child: Icon(
                      Icons.arrow_back,
                      color: Colors.white,
                      size: 40.0,
                    ),
                  ),
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

flutter:
  assets:
    - assets/background_image.jpg