Fix "docker manifest create" issue

This patch resolves the issue that causes Docker to fail when
it runs the "docker manifest create" command since the registry
does not mention that it supports the 2.0 API inside it's
headers.

Change-Id: I777bd4218c822fa665f0d29f573a6ae120194284

playbooks/functional-test/docker.yaml

@@ -73,10 +73,6 @@
 
 - name: Create a manifest with multi architecture images
   command: docker manifest create localhost:9000/alpine:3 localhost:9000/alpine:3-amd64 localhost:9000/alpine:3-arm64
-  # NOTE(mnaser): This should fail because of the following bug:
-  #               https://github.com/docker/cli/issues/3161
-  register: result
-  failed_when: result.rc != 1
 
 - name: Wipe all images from the local cache (tagged in DockerHub)
   command: docker rmi docker.io/library/alpine@{{ item.value }}

zuul_registry/main.py

@@ -72,6 +72,13 @@ class Authorization(cherrypy.Tool):
         return store[user] == password
 
     def unauthorized(self, scope):
+        # NOTE(mnaser): This is to workaround the following Docker CLI bug and
+        #               should be removed once it is fixed:
+        #
+        #               https://github.com/docker/cli/issues/3161
+        cherrypy.response.headers['Docker-Distribution-Api-Version'] = (
+            'registry/2.0'
+        )
         cherrypy.response.headers['www-authenticate'] = (
             'Bearer realm="%s/auth/token",scope="%s"' % (
                 self.public_url, scope)