Rootless server deployment with no dependencies

[richiejp]

The server currently has three dependencies: Containerd, Buildkitd and Nerdctl. It also must run as root because Nerdctl fails to create a bridge device in rootless mode (possibly due to detach netns https://github.com/containerd/nerdctl/blob/main/docs/rootless.md#rootlesskit-network-design).

As default we want to run the server as a non-root user and not have to install or configure anything else. The user can then quickly try Ayup without messing around with these dependencies that have a lot of sharp edges or giving us root access.

[richiejp]

Now need to bundle Rootlesskit, Buildkit and the CNI plugins, plus any exe's that these happen to use, but hopefully they are reasonably self contained.

[richiejp]

Go very close to this with #35

Nix bundle almost works, but it creates a new user namespace (IIUC) and some capability needed by Rootlesskit gets dropped. However we maybe can patch the bundler to work around this.