you can’t use bench get-app in production images.
you need to build images with your app and use those images.
I can’t help any more than the docs: frappe_docker/README.md at main · frappe/frappe_docker · GitHub or example: GitHub - castlecraft/custom_frappe_docker
somehow it ONLY works for me. It built the images here Packages · castlecraft · GitHub. I can’t help any further.