The automatic creation of review apps is one of the biggest advantages of using Heroku. But what about using files in your S3 bucket? I've build a Symfony console command to upload files automatically to Cloudcube through flysystem.
The command receives a path to a directory with fixture files and copies them over to the S3 storage. Although the command is structured to not only work with S3 but every flysystem storage configuration you supply to it.
I've set the directory to /heroku/review/storage and added the following line to /heroku/review/setup-review.sh:
Within my app.json file I trigger the setup-review.sh script in the postdeploy process.
This process is run after the release phase of the review app but only ones after the initial build of the review app.