see https://systemd.io/PORTABLE_SERVICES/ about the definition of
portable services. this tooling is analogous to the `pkgs.dockerTools.buildImage`
tooling and is called `pkgs.portableService`.
systemd (since version 239) supports a concept of “Portable Services”.
“Portable Services” are a delivery method for system services that uses
two specific features of container management:
* Applications are bundled. I.e. multiple services, their binaries and all
their dependencies are packaged in an image, and are run directly from it.
* Stricter default security policies, i.e. sandboxing of applications.
The primary tool for interacting with Portable Services is portablectl,
and they are managed by the systemd-portabled service.
This function will create a squashfs raw image in `result/$pname_$version.raw`
that has the required files by the portable services spec, and all the
dependencies for the running program in the nix store.
