Make docker.nix match Nixpkgs's idioms

1. `target` is the wrong name, that is just for compilers per out
standard terminology. We just need to worry about "build" and "host".

2. We only need one `pkgs`. `pkgs.buildPackages` is how we get anything
we need at build time.

3. `crossSystem` is the name of a nixpkgs parameter that is actually an
attribute set, not a 2-part "cpu-os" string.

3. `pkgsCross` effectively evaluates Nixpkgs twice, which is
inefficient. It is just there for people poking around the CLI / REPL
(and I am skeptical even that is a good idea), and *not* what written
code should use, especially code that is merely parametric in the package set
it is given.

4. We don't need to memoize Nixpkgs here because we are only doing one
pkg set at a time (no `genAttrs`) so it's better to just delete all this
stuff. `flake.nix` instead would do something like that, with
`genAttrs` (though without `pkgsCross`), if and when we have hydra jobs
for cross builds.

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