Motivation for this change

I've been seeing issues where existing preloaded images in the docker daemon gets garbled when restarting docker.service. This happens because dockerPreloader overwrites repository.json and it is not designed to preserve any images that might be pre-loaded before it runs.

example:

Currently, there's no way of disabling the dockerPreloader. But I think it is ok to assume that if the list of images to preload is empty, we can safely drop configuring the systemd unit for the preloader. What do you say @nlewo?

cc @srhb

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.