Google App Engine写入权限被拒绝

I have uploaded my OctoberCMS (Laravel 5 based CMS) to Flexible Google App Engine using following console command:

╭─yusuf@berkarya ~/htdocs/Project_October 
╰─➤  gcloud app deploy 

And then it deployed to the Cloud Service with following messages:

Updating service [default] (this may take several minutes)...done.                                                                                                      
Setting traffic split for service [default]...done.                                                                                                                     
Stopping version [obundaoctober/default/20180614t144216].
Sent request to stop version [obundaoctober/default/20180614t144216]. This operation may take some time to complete. If you would like to verify that it succeeded, run:
  $ gcloud app versions describe -s default 20180614t144216
until it shows that the version has stopped.
Deployed service [default] to [https://obundaoctober.appspot.com]

You can stream logs from the command line by running:
  $ gcloud app logs tail -s default

To view your application in the web browser run:
  $ gcloud app browse

The problem is when I tried open following url https://obundaoctober.appspot.com I got an error message that inform me it has no sufficient permission access:

We're sorry, but an unhandled error occurred. Please see the details below.

The stream or file "/app/storage/logs/system.log" could not be opened: failed to open stream: Permission denied
/app/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php line 107

Any proper way to change folder/file permission in flexible Google App Engine to solve this kind of error ?

Any help will be extremely appreciated.

Thanks

In the GAE tutorial for Laravel is an example to change permissions in the post-install-cmd in composer.json

"scripts": {
    "post-install-cmd": [
        "chmod -R 755 bootstrap\/cache",
        "php artisan cache:clear"
    ]
}

I think this will also work with the storage folder.