Provide Dockerfile target for pre-cached vm & vm-rootless

[elpdt852]

This will let users without Nix to try out nix-snapshotter with just:

# Entrypoint launches directly into `.#vm` QEMU
docker run --rm -it ghcr.io/pdtpartners/nix-snapshotter

# Entrypoint launches directly into `.#vm-rootless` QEMU
docker run --rm -it ghcr.io/pdtpartners/nix-snapshotter:rootless

I'm not adding a CD target atm because this requires a docker builder, and a rather long build without caching capability. Though not impossible because I believe buildkit allows you to store caches in an OCI image, so we may be able to push it to ghcr.io.

[RobbieBuxton]

Looks good, one thing I've been wondering, which is a bit unrelated but is there any particular reason why throughout the repo we are using/recommending nix run ".#vm" as opposed to nix run .#vm. e.g in this pr "#nixosConfigurations.vm.config.system.build.vm" instead of .#nixosConfigurations.vm.config.system.build.vm .
From my understanding the quotation marks aren't necessary as long as you include a period before?

[elpdt852]

In some shells like zsh in certain modes, the # means something special to the shell so it actually errors. Wrapping it in quotes ensures it works across all shells.

See: NixOS/nix#4686

[RobbieBuxton]

In some shells like zsh in certain modes, the # means something special to the shell so it actually errors. Wrapping it in quotes ensures it works across all shells.

See: NixOS/nix#4686

Makes sense, cheers!