It's not exactly what you're asking for as it relates to running containers via runc. But this walks through how OCI layers are actually built up behind the scenes. In case it's helpful: https://depot.dev/blog/building-container-layers-from-scratc...
Anyway, fortunately I seem to have found a solution for now (running runc with an overlay rootfs without root), see the link in the other sibling/nephew comment I posted.
One example: https://ravichaganti.com/blog/2022-11-28-building-container-...
> I got familiar with container intervals via, basically, "building images without docker"
That's been my approach, as well. However, right now I'm stuck at getting overlayfs to work without privileges (easy: use `unshare`), while not breaking rootless runc (apparently not so easy).
https://github.com/psanford/runc-examples/blob/master/rootle...
You can also use docker save to get a tarball and ship that file to another machine, which can be run through docker load and then run as if it was built locally.
If you have an oci bundle, you might look at runc instead: https://github.com/opencontainers/runc