Previously this module precluded use of storage backends other than
`filesystem`. It is now possible to configure another storage backend
manually by setting `services.dockerRegistry.storagePath` to `null` and
configuring the other backend via `extraConfig`.
The following changes have been applied:
- the property `http.headers.X-Content-Type-Options` must a list of
strings rather than a serialized list
- instead of `/etc/docker/registry/config.yml` the configuration will be
written with `pkgs.writeText` and the store path will be used to run
the registry. This reduces the risk of possible impurities by relying
on the Nix store only.
- cleaned up the property paths to easy readability and reduce the
verbosity.
- enhanced the testcase to ensure that digests can be deleted as well
- the `services.docker-registry.extraConfig` object will be merged with
`registryConfig`
/cc @ironpinguin
