Bikeshedding aside, I’m assuming what you actually want is to run snaps inside of a Docker container? systemd is not a blocker for “Docker” support. It’s definitely possible to run systemd in OCI containers, Podman even has native support for it.
For reference; these are some of the earlier discussions about snapd in Docker.
I haven’t found a clear statement yet whether Docker support is out of the question entirely, so it would be nice to have someone from the snapd team comment on this.
Some level of Docker support would be nice, even if the snaps are unconfined, given that many build systems run builds inside of Docker containers and snapcraft has required snap support for a very long time.