Tart is great! This is probably the best thing available for now, though it runs into some limitations that Apple imposes for VMs. (Those limitations perhaps hint at why Apple hasn't implemented this-- it seems they don't really want people to be able to rent out many slices of Macs.

One clever and cool thing Tart actually does that sort of relates to this discussion is that it uses the OCI format for distributing OS images!

(It's also worth noting that Tart is proprietary. Some users might prefer something that's either open-source, built-in, or both.)