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; }

docker: allow overriding package inputs per docker package