sysbox: Permission denied when mounting "proc" to rootfs at "/proc" with docker-in-docker

I’m trying out sysbox 0.4.1 on the following host, to try and get docker-in-docker running:

• Docker 20.10.12
• Ubuntu 20.04
• Kernel 5.11.0-27-generic (HWE)

When I follow the first example in the quickstart section, I get the following error:

docker: Error response from daemon: failed to create shim: OCI runtime create failed:
container_linux.go:380: starting container process caused:
process_linux.go:545: container init caused:
rootfs_linux.go:76: mounting "proc" to rootfs at "/proc" caused:
mount through procfd: permission denied: unknown.

This happens inside the system container. I tried running the system container with --privileged, but the result remains the same.

Here’s the complete output:

$ docker run --runtime=sysbox-runc -it --hostname=syscont nestybox/alpine-docker:latest
$ dockerd > /var/log/dockerd.log 2>&1 &
$ docker run -it busybox
Unable to find image 'busybox:latest' locally
latest: Pulling from library/busybox
5cc84ad355aa: Pull complete
Digest: sha256:5acba83a746c7608ed544dc1533b87c737a0b0fb730301639a0179f9344b1678
Status: Downloaded newer image for busybox:latest
docker: Error response from daemon: failed to create shim: OCI runtime create failed: container_linux.go:380: starting container process caused: process_linux.go:545: container init caused: rootfs_linux.go:76: mounting "proc" to rootfs at "/proc" caused: mount through procfd: permission denied: unknown.
ERRO[0004] error waiting for container: context canceled