Since https://github.com/NixOS/nixpkgs/pull/61321, local-fs.target is
part of sysinit.target again, meaning units without
DefaultDependencies=no will automatically depend on it, and the manual
set dependencies can be dropped.
This change adds a simple integration test exercising the fetchdocker
Nix code and hocker utilities for the simple `hello-world` docker
container. We exercise:
- Fetching the docker image configuration json
- Fetching the docker image layers
- Building a compositor script
- Loading the `hello-world` docker image into docker using the
compositor script and `docker load`
- Running that loaded container
