There’s literally not much to say about the scratch container as it’s completely empty! This container is usually only used when creating a base container from an external root filesystem in combination with the ADD command.
A Dockerfile that does this would look like:
FROM scratch ADD rootfs.tar /
The root filesystem can be created outside of Docker or downloaded from a third party. For example when creating a base container using Debian the root filesystem can be created with debootstrap. The ADD command takes the tarball and extracts it into the container.