default-data directory is where we store your application files for your custom image. This is done so that when you access your container via SSH/SFTP we can mount the same directories as your container.
Any non-volatile data should be stored in
default-data, if you store your application inside the image (which we don't recommend) when you remove a container, or swap image versions that data will be gone. Any directory inside the default-data directory can be mounted to your custom image with the use of the manifest.yml file.
It's worth noting that all data stored inside the
default-datadirectory will have its permissions overwritten to UID 33 (www-data) and GID 33 (www-data), we recommend that any application that has permission specific requirements use the same permissions.