Changes the return format of Nixery's build procedure to return a JSON
structure that can indicate which errors have occured.
The server can use this information to send appropriate status codes
back to clients.
This extends the package set import mechanism in
build-registry-image.nix with several different options:
1. Importing a nixpkgs channel from Github (the default, pinned to
nixos-19.03)
2. Importing a custom Nix git repository. This uses builtins.fetchGit
and can thus rely on git/SSH configuration in the environment (such
as keys)
3. Importing a local filesystem path
As long as the repository pointed at is either a checkout of nixpkgs,
or nixpkgs overlaid with custom packages this will work.
A special syntax has been defined for how these three options are
passed in, but users should not need to concern themselves with it as
it will be taken care of by the server component.
This relates to #3.
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.
