Fix dependency proxy image prefix

Changelog: fixed

app/models/group.rb

@@ -276,7 +276,7 @@ class Group < Namespace
   def dependency_proxy_image_prefix
     # The namespace path can include uppercase letters, which
     # Docker doesn't allow. The proxy expects it to be downcased.
-    url = "#{web_url.downcase}#{DependencyProxy::URL_SUFFIX}"
+    url = "#{Gitlab::Routing.url_helpers.group_url(self).downcase}#{DependencyProxy::URL_SUFFIX}"
 
     # Docker images do not include the protocol
     url.partition('//').last

spec/models/group_spec.rb

@@ -2756,6 +2756,10 @@ RSpec.describe Group do
     it 'removes the protocol' do
       expect(group.dependency_proxy_image_prefix).not_to include('http')
     end
+
+    it 'does not include /groups' do
+      expect(group.dependency_proxy_image_prefix).not_to include('/groups')
+    end
   end
 
   describe '#dependency_proxy_image_ttl_policy' do