在前端网站读取public下的图片资源，存在跨域问题，不懂在哪设置才能前端网站才能访问。
前端网站读取报以下错误
Access to Image at 'http://www.a.com/mat_image/a9/3ec0a8f154461af7d06cfa317b4303.png' from origin 'http://www.b.com' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://www.b.com' is therefore not allowed access.