nixos/docker-registry: allow nested config options

for example:

services.dockerRegistry = {
    enable = true;
    extraConfig = {
        http = {
            host = "https://${config.networking.hostName}:5000";

            tls = {
                certificate = "${registry-tls}/snakeoil.pem";
                key = "${registry-tls}/snakeoil.key";
            };
        };
    };
};