Updating configuration with required header

Signed-off-by: Mary Anthony <mary@docker.com>

contrib/compose/nginx/registry.conf

@@ -25,6 +25,9 @@ server {
     if ($http_user_agent ~ "^(docker\/1\.(3|4|5(?!\.[0-9]-dev))|Go ).*$" ) {
       return 404;
     }
+    
+    # The docker client expects this header from the /v2/ endpoint.
+    more_set_headers 'Docker-Distribution-Api-Version: registry/2.0';
 
     include               docker-registry-v2.conf;
   }