Make Image CR stager upgrade to CustomBuilder

- This change supports upgrades from cf-for-k8s versions with the
Builder to versions with the new CustomBuilder (cf-default-builder).

[finishes #173536510]
Co-authored-by: Piyali Banerjee <pbanerjee@pivotal.io>
Co-authored-by: Connor Braa <cbraa@pivotal.io>

Author: piyalibanerjee

lib/cloud_controller/kpack/stager.rb

@@ -1,5 +1,9 @@
 module Kpack
   class Stager
+    CF_DEFAULT_BUILDER = {
+      name: 'cf-default-builder',
+      kind: 'CustomBuilder'
+    }.freeze
     APP_GUID_LABEL_KEY = 'cloudfoundry.org/app_guid'.freeze
     BUILD_GUID_LABEL_KEY = 'cloudfoundry.org/build_guid'.freeze
     STAGING_SOURCE_LABEL_KEY = 'cloudfoundry.org/source_type'.freeze
@@ -47,6 +51,7 @@ def update_image_resource(image, staging_details)
       image.metadata.labels[BUILD_GUID_LABEL_KEY.to_sym] = staging_details.staging_guid
       image.spec.source.blob.url = blobstore_url_generator.package_download_url(staging_details.package)
       image.spec.build.env = get_environment_variables(staging_details)
+      image.spec.builder = CF_DEFAULT_BUILDER
 
       image
     end
@@ -67,10 +72,7 @@ def image_resource(staging_details)
         },
         spec: {
           serviceAccount: registry_service_account_name,
-          builder: {
-            name: 'cf-default-builder',
-            kind: 'CustomBuilder'
-          },
+          builder: CF_DEFAULT_BUILDER,
           tag: "#{registry_tag_base}/#{staging_details.package.app.guid}",
           source: {
             blob: {

spec/unit/lib/cloud_controller/kpack/stager_spec.rb

@@ -131,8 +131,8 @@ module Kpack
               tag: "gcr.io/capi-images/#{package.app.guid}",
               serviceAccount: 'gcr-service-account',
               builder: {
-                name: 'cf-default-builder',
-                kind: 'CustomBuilder'
+                name: 'cf-autodetect-builder', # legacy Builder to verify that image update includes new CustomBuilder
+                kind: 'Builder'
               },
               source: {
                 blob: {
@@ -161,6 +161,8 @@ module Kpack
           updated_image.spec.build.env = [
             { name: 'FOO', value: 'BAR' }
           ]
+          updated_image.spec.builder.kind = 'CustomBuilder'
+          updated_image.spec.builder.name = 'cf-default-builder'
 
           expect(client).to_not receive(:create_image)
           expect(client).to receive(:update_image).with(updated_image)