Question: Supporting modifications to a read-only rootfs

(This is more of a question, but I wasn't sure where to post this other than the issue tracker.)

I'm trying to use `crun` as the back-end for a sandboxing tool that makes it possible to modify a container's root filesystem, optionally persisting the changes into a separate user directory. I'm having some trouble mapping that onto an OCI bundle that works with `crun.

First of all, as per the spec the `root` key is mandatory, but since I don't actually want to mutate the sources of the `rootfs` I'm setting `readonly=true`.

I figured I could then (kind of redundantly) mount the root again, but now using an `overlay` mount to persist the changes:

```json
"root": {
"path": "/rootfs",
"readonly": true
},
"mounts": [
{
"destination": "/",
"type": "overlay",
"options": [
"lowerdir=/rootfs",
"upperdir=/overlay/upper",
"workdir=/overlay/work",
"userxattr"
]
},
]
```

`crun` doesn't accept that, however. Well, more specifically the overlay mount fails with `EINVAL`.

Looking at a bundle generated by Docker, which supports this, the `root` is actually writable and pointing to e.g. a btrfs subvolume. I would like to avoid that, because it requires the caller to set-up an overlay-like mechanism to make the root writable, while I'd prefer `crun` to handle all that (with the added benefit of easy clean-up when the container exits). Is this possible, or outside of the scope of `crun`? Any other suggestions would be appreciated as well!