Introduce an empty runtime configuration object in each built layer.
This is required because Kubernetes expects the configuration to be
present (even if it's just empty values).
Providing an empty configuration will make Docker's API return a full
configuration struct with default (i.e. empty) values rather than
`null`, which works for Kubernetes.
This fixes issue #1. See the issue for additional details.
Instead of using whatever the current system default is, import a Nix
channel when building an image.
This will use Nix' internal caching behaviour for tarballs fetched
without a SHA-hash.
For now the downloaded channel is pinned to nixos-19.03.
