You are probably referring to Sysbox (https://github.com/nestybox/sysbox), which I believe will meet your requirements (systemd, inner containers, security, etc).

Btw, Sysbox is already supported in Docker-Desktop (business tier only), so you can easily do what you want with this instruction:

``` $ docker run -it --rm -e SYSBOX_SYSCONT_MODE=TRUE ghcr.io/nestybox/ubuntu-focal-systemd-docker:latest bash ```

Disclaimer: I'm Sysbox's co-creator and currently working for Docker.