Data images add containers with the given image as sidekick to all services, adding the to the volumes-from list.
One possible use case for this is requiring tls certificates to connect to a mysql server. Build an image with the images in /mysql/certificates and VOLUME on /mysql/certificates. Then add the the (private repostory) image as data-image.
-> Your app will find the certificates in /mysql/certificates
{
"data-images":[
"registry.gitlab.com/organization/image:version"
]
}