Actually in my experience with pulling very large images to run with docker it turns out that Docker doesn't really do any fsync-ing itself. The sync happens when it creates an overlayfs mount while creating a container because the overlayfs driver in the kernel does it.

A volatile flag to the kernel driver was added a while back, but I don't think Docker uses it yet https://www.redhat.com/en/blog/container-volatile-overlay-mo...