dockerTools: add nix-prefetch-docker script

[offlinehacker]

Motivation for this change

Have a sane way to get output hash for dockerTools.pullImage

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 nix-review --run "nix-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.

---

[nlewo]

Yeah! Actually I'm working on something quite similar: nmattia/niv#34 ;)

I think we could expose this derivation in all-packages.nix. Something such as:

nix-prefetch-docker = callPackage build-support/docker/nix-prefectch-docker.nix {}

Otherwise I tried it and it's working fine.

[offlinehacker]

@nlewo i applied your suggestions

[offlinehacker]

I also updated docs

[nlewo]

Thanks!