disallow group runners to become project runners

lib/api/runners.rb

@@ -206,6 +206,7 @@ module API
       def authenticate_enable_runner!(runner)
         forbidden!("Runner is shared") if runner.is_shared?
         forbidden!("Runner is locked") if runner.locked?
+        forbidden!("Runner is a group runner") if runner.group?
         return if current_user.admin?
 
         forbidden!("No access granted") unless user_can_access_runner?(runner)

spec/requests/api/runners_spec.rb

@@ -658,6 +658,12 @@ describe API::Runners do
         expect(response).to have_gitlab_http_status(403)
       end
 
+      it 'does not enable group runner' do
+        post api("/projects/#{project.id}/runners", user), runner_id: group_runner.id
+
+        expect(response).to have_http_status(403)
+      end
+
       context 'user is admin' do
         it 'enables any specific runner' do
           expect do