Fix :container_expiration_policy Import

`:container_expiration_policy` was not imported as was not added into
UNIQUE_RELATIONS and EXISTING_OBJECT_CHECK hashes.
These hashes are very implicit and undiscoverable for the developer
who is adding new relation so they don't even know that the Import will
not be succeed.

lib/gitlab/import_export/relation_factory.rb

@@ -38,12 +38,12 @@ module Gitlab
 
       IMPORTED_OBJECT_MAX_RETRIES = 5.freeze
 
-      EXISTING_OBJECT_CHECK = %i[milestone milestones label labels project_label project_labels group_label group_labels project_feature merge_request ProjectCiCdSetting].freeze
+      EXISTING_OBJECT_CHECK = %i[milestone milestones label labels project_label project_labels group_label group_labels project_feature merge_request ProjectCiCdSetting container_expiration_policy].freeze
 
       TOKEN_RESET_MODELS = %i[Project Namespace Ci::Trigger Ci::Build Ci::Runner ProjectHook].freeze
 
       # This represents all relations that have unique key on `project_id`
-      UNIQUE_RELATIONS = %i[project_feature ProjectCiCdSetting].freeze
+      UNIQUE_RELATIONS = %i[project_feature ProjectCiCdSetting container_expiration_policy].freeze
 
       def self.create(*args)
         new(*args).create

spec/fixtures/lib/gitlab/import_export/complex/project.json

@@ -6757,6 +6757,17 @@
       "updated_at": "2017-01-16T15:25:29.637Z"
     }
   ],
+  "container_expiration_policy": {
+    "created_at": "2019-12-13 13:45:04 UTC",
+    "updated_at": "2019-12-13 13:45:04 UTC",
+    "next_run_at": null,
+    "project_id": 5,
+    "name_regex": null,
+    "cadence": "3month",
+    "older_than": null,
+    "keep_n": 100,
+    "enabled": false
+  },
   "deploy_keys": [],
   "services": [
     {

spec/lib/gitlab/import_export/project_tree_restorer_spec.rb

@@ -240,6 +240,16 @@ describe Gitlab::ImportExport::ProjectTreeRestorer do
         expect(sentry_issue.sentry_issue_identifier).to eq(1234567891)
       end
 
+      it 'restores container_expiration_policy' do
+        policy = Project.find_by_path('project').container_expiration_policy
+
+        aggregate_failures do
+          expect(policy).to be_an_instance_of(ContainerExpirationPolicy)
+          expect(policy).to be_persisted
+          expect(policy.cadence).to eq('3month')
+        end
+      end
+
       context 'Merge requests' do
         it 'always has the new project as a target' do
           expect(MergeRequest.find_by_title('MR1').target_project).to eq(@project)