fix: do not add the cache access policy if there is none (#540)

* do not add a policy if we have none

* consider manual ARN

Author: kayman-mk

Diff for: main.tf

@@ -390,6 +390,8 @@ resource "aws_iam_role_policy_attachment" "user_defined_policies" {
 ### Policy for the docker machine instance to access cache
 ################################################################################
 resource "aws_iam_role_policy_attachment" "docker_machine_cache_instance" {
+  count = var.cache_bucket["create"] || length(lookup(var.cache_bucket, "policy", "")) > 0 ? 1 : 0
+
   role       = aws_iam_role.instance.name
   policy_arn = local.bucket_policy
 }