app: add helper function to computer a group's Dependency Proxy URL

app/helpers/groups_helper.rb

@@ -99,6 +99,12 @@ module GroupsHelper
       .count
   end
 
+  def group_dependency_proxy_url(group)
+    # The namespace path can include uppercase letters, which
+    # Docker doesn't allow. The proxy expects it to be downcased.
+    "#{group_url(group).downcase}#{DependencyProxy::URL_SUFFIX}"
+  end
+
   def group_icon_url(group, options = {})
     if group.is_a?(String)
       group = Group.find_by_full_path(group)

app/views/groups/dependency_proxies/_url.html.haml

--# The namespace path can include uppercase letters, which
--# Docker doesn't allow. The proxy expects it to be downcased.
-- proxy_url = "#{group_url(@group).downcase}#{DependencyProxy::URL_SUFFIX}"
+- proxy_url = group_dependency_proxy_url(@group)
 
 %h5.prepend-top-20= _('Dependency proxy URL')
 

spec/helpers/groups_helper_spec.rb

@@ -23,6 +23,13 @@ RSpec.describe GroupsHelper do
     end
   end
 
+  describe 'group_dependency_proxy_url' do
+    it 'converts uppercase letters to lowercase' do
+      group = create(:group, path: 'GroupWithUPPERcaseLetters')
+      expect(group_dependency_proxy_url(group)).to end_with("/groupwithuppercaseletters#{DependencyProxy::URL_SUFFIX}")
+    end
+  end
+
   describe 'group_lfs_status' do
     let(:group) { create(:group) }
     let!(:project) { create(:project, namespace_id: group.id) }