nixos/docker-registry: cleanup module definition & enhance testcase

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

nixos/tests/docker-registry.nix

@@ -3,12 +3,13 @@
 import ./make-test.nix ({ pkgs, ...} : {
   name = "docker-registry";
   meta = with pkgs.stdenv.lib.maintainers; {
-    maintainers = [ globin ];
+    maintainers = [ globin ma27 ];
   };
 
   nodes = {
     registry = { config, pkgs, ... }: {
       services.dockerRegistry.enable = true;
+      services.dockerRegistry.enableDelete = true;
       services.dockerRegistry.port = 8080;
       services.dockerRegistry.listenAddress = "0.0.0.0";
       networking.firewall.allowedTCPPorts = [ 8080 ];
@@ -22,6 +23,7 @@ import ./make-test.nix ({ pkgs, ...} : {
     client2 = { config, pkgs, ...}: {
       virtualisation.docker.enable = true;
       virtualisation.docker.extraOptions = "--insecure-registry registry:8080";
+      environment.systemPackages = [ pkgs.jq ];
     };
   };
 
@@ -39,5 +41,9 @@ import ./make-test.nix ({ pkgs, ...} : {
     $client2->waitForUnit("docker.service");
     $client2->succeed("docker pull registry:8080/scratch");
     $client2->succeed("docker images | grep scratch");
+
+    $client2->succeed(
+      'curl -fsS -X DELETE registry:8080/v2/scratch/manifests/$(curl registry:8080/v2/scratch/manifests/latest | jq ".fsLayers[0].blobSum" | sed -e \'s/"//g\')'
+    );
   '';
 })