Update terraform to support terragrunt builder

deployment/modules/gcp/cloudbuild/main.tf

@@ -17,6 +17,29 @@ terraform {
   backend "gcs" {}
 }
 
+# This is the target registry for the terragrunt community builder image.
+# Note that it needs to have location "us" rather than var.region as this
+# is hard-coded in the builder config.
+resource "google_artifact_registry_repository" "terragrunt" {
+  repository_id = "terragrunt"
+  location      = "us"
+  description   = "Terragrunt builder image"
+  format        = "DOCKER"
+}
+
+data "google_compute_default_service_account" "default" {
+}
+
+resource "google_artifact_registry_repository_iam_binding" "binding" {
+  project = google_artifact_registry_repository.terragrunt.project
+  location = google_artifact_registry_repository.terragrunt.location
+  repository = google_artifact_registry_repository.terragrunt.name
+  role = "roles/artifactregistry.writer"
+  members = [
+    google_compute_default_service_account.default.member,
+  ]
+}
+
 resource "google_artifact_registry_repository" "docker" {
   repository_id = "docker-${var.env}"
   location      = var.region
@@ -82,7 +105,7 @@ resource "google_cloudbuild_trigger" "docker" {
     // for instructions.
     step {
       id   = "terraform_plan_all"
-      name = "cloud-builders-community/terragrunt"
+      name = "gcr.io/${var.project_id}/terragrunt"
       args = ["plan-all"]
       env = [
         "TF_IN_AUTOMATION=1",