…stry

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

…gistry

nixos/docker-registry: add configuration options to support image deletions