Introduces three new types representing each of the possible package
sources and moves the logic for specifying the package source to the
server.
Concrete changes:
* Determining whether a specified git reference is a commit vs. a
branch/tag is now done in the server, and is done more precisely by
using a regular expression.
* Package sources now have a new `CacheKey` function which can be used
to retrieve a key under which a build manifest can be cached *if*
the package source is not a moving target (i.e. a full git commit
hash of either nixpkgs or a private repository).
This function is not yet used.
* Users *must* now specify a package source, Nixery no longer defaults
to anything and will fail to launch if no source is configured.
Moves the relevant parts of the customisation layer construction from
dockerTools.mkCustomisationLayer into the Nixery code base.
The version in dockerTools builds additional files (including via
hashing of potentially large files) which are not required when
serving an image over the registry protocol.
Some upcoming changes might require the Nix build to be split into
multiple separate nix-build invocations of different expressions, thus
splitting this out is useful.
It also fixes an issue where `build-image/default.nix` might be called
in an environment where no Nix channels are configured.
ALl the ones except for build-image.nix are considered trivial. On the
latter, nixfmt makes some useful changes but by-and-large it is not
ready for that code yet.
Removes usage of the old layering algorithm and replaces it with the
new one.
Apart from the new layer layout this means that each layer is now
built in a separate derivation, which hopefully leads to better
cacheability.
Instead of requiring the server component to be made aware of the
location of the Nix builder via environment variables, this commit
introduces a wrapper script for the builder that can simply exist on
the builders $PATH.
This is one step towards a slightly nicer out-of-the-box experience
when using `nix-build -A nixery-bin`.
