So in my template, I have created the VMs with two disks: first one is for NixOS and is built from an image, and it is read-only. The second is mounted to /var and is used for all system configuration as well as application state. If I have multiple VMs, they can all share the same base image (thin provisioned). That's the mode that I want for my deployments of services, immutable and as stateless as possible. For agent use, its different, you actually want a mutable NixOS root so that the agent can do what it wants.
I built three modes: immutable, semi-mutable, and mutable. mutable removes the read-only lock on the root, and just lets you manage the VM as a pet. semi-mutable adds an ephemeral overlayfs that gets wiped the next time you upgrade the base image. So that gives you kind of the best of both worlds: an immutable read-only base image and the ability to "nix profile add" whatever you (or your agent) wants, but with the contract that these imperatively installed things will disappear the next time you upgrade. Are you planning on adding a LICENSE to your machine0-nixos repo?
I mean, I'm not going to claim it's remotely near the same quality. And proxmox has some holes in their support for cloud init. And of course you need a mini pc on a good internet connection or the like.
But extremely fast provisioning of a any of VMs ... very handy.
Proxmox has too many compromises though. Maybe I should do the reverse, and extend this until it can fully replace proxmox entirely.
exe.dev is great but lurking in my mind is: "how will I replicate this if I ever need to move to AWS etc.." for all the service composition.
Site looks great too