[Feature request] MacOS Images

**Context**

Right now `unity-builder` is using MacOS runners to install Unity Editor on the fly.
The reason for it is that MacOS doesn't run from docker images. Or didn't...

**Suggested solution**

Since then there's been a fair bit of development.

Collecting links here:

**Discussion**
- https://github.com/containerd/containerd/discussions/5525
- https://stackoverflow.com/questions/40824547/docker-image-running-a-mac-os-x-installation

**Possible solutions**
- https://github.com/sickcodes/Docker-OSX
- https://github.com/darwin-containers

We should at some point decide that docker for MacOS is good enough and pick one of the technologies.

**Considered alternatives**

Keep using on the fly Unity Editor installs.

**Additional details**