… store

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!

The database dump doesn't contain sha and size. This leads to invalid
path in the container. We have to fix the database by using
nix-store.
Note a better way to do this is available in Nix 1.12 (since the
database dump contains all required information).

We also add content output paths in the gcroots since they ca be used
by the container.

dockerTools: populate the nix database of the container nix store