Previously we could not override the input packages to docker at all as
that had been shadowed due to the usage of inherit in all-package.nix.

We are now mixing the "variable" variables for the build and the
dependency declarations for the package and using callPackage as if each
of the docker packages would be separate files that we evaluate.

This now enables overriding e.g. iptables for a single verison of
docker which previously was not feasible without importing the
default.nix again (and passing different arguments).

pkgs.docker.override { iptables = pkgs.iptables-nftables-compat; }

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.