`docker pull bitnami/redis:7.0` fails with EPERM

> We'd like to basically capture and record the filesystem state of the sandboxee (merged view of upper and lower layer, or just the upper layer contents).
>
> You want to capture the rootfs overlay's upper layer contents? Or are you talking about checkpoint/restore with gVisor (which should already work with all modes of --overlay2 flag)?
>
> Aiming to do the former, but at the same time need to use overlay2 because we're trying to run docker inside gvisor and docker doesn't appear work inside gVisor with `--overlay2=none` due to `failed to register layer: operation not permitted` when trying to pull certain images, which I think is due to restrictions of the 9p filesystem? (e.g., `docker pull bitnami/redis:7.0` which doesn't work when root filesystem shows `9p`, but works when it shows `overlay`)
>
> We currently don't have a way to extract just the rootfs upper layer from the checkpoint image generated by runsc checkpoint. I am (co-incidentally) working on extracting the rootfs upper layer as a TAR file. Would you be interested in that? We could add some API in runsc to invoke that functionality.
>
> Would be interested in that!

_Originally posted by @xpbowler in [#11892](https://github.com/google/gvisor/issues/11892#issuecomment-3046603254)_