调用云端云函数，报错：FUNCTIONS_EXECUTE_FAIL，"Error: EROFS: read-only file system, open '/var/user/xxx'"

想利用云函数实现将图片转为GIF，然后返回的功能，在本地测试ok,但是上传云函数后调用，返回报错：
请问这是没有权限操作文件的原因吗？该如何解决或有没有其他办法可以实现这个功能？

code: "FUNCTIONS_EXECUTE_FAIL"  
requestId: "c6d7c69bbdf27"

服务器报错：

2022-07-20T14:16:52.773Z START  
2022-07-20T14:16:52.953Z "/var/user" "/var/user"  
2022-07-20T14:16:52.962Z "Error: EROFS: read-only file system, open '/var/user/out.gif'"  
2022-07-20T14:16:53.455Z "readable:"  
2022-07-20T14:16:55.666Z REPORT RequestId:fc6adf7b-fd91-4382-b695-31e19e9d009d Duration:2398ms Memory:256MB MemUsage:62.400192MB  
2022-07-20T14:16:55.666Z END  
2022-07-20T14:16:55.666Z ERROR RequestId:fc6adf7b-fd91-4382-b695-31e19e9d009d Result:{"errCode":1,"errMsg":"user code exception caught","stackTrace":"ENOENT: no such file or directory, open '/var/user/out.gif'  
Error: ENOENT: no such file or directory, open '/var/user/out.gif'  
    at Object.fs.openSync (fs.js:646:18)  
    at Object.fs.readFileSync (fs.js:551:33)  
    at Object.exports.main (/var/user/__index.js:77:18)  
    at \u003canonymous\u003e  
    at process._tickCallback (internal/process/next_tick.js:188:7)"}