Currently, the contents closure is copied to the layer but there is no
nix database initialization. If pkgs.nix is added in the contents,
nix-store doesn't work because there is no nix database.
From the contents of the layer, this commit generates and loads the
database in the nix store of the container. This only works if there
is no parent layer that already have a nix store (to support several
nix layers, we would have to merge nix databases of parent layers).
We also add an example to play with the nix store inside the
container. Note it seems `more` is a missing dependency of the nix
package!
Before this patch, a VM was used to spawn docker that pulled the
VM. Now, the tool Skopeo does this job well so we can simplify our
dockerTools since we doesn't need Docker anymore:)
This also fixe the regression described in
https://github.com/NixOS/nixpkgs/issues/29271 : cntlm proxy doesn't
work in 17.09 while it worked in 17.03.
Note Skopeo doesn't produce the same output than docker pull so, we
have to update sha.
