fix: allow changes to "runner_worker.max_jobs" for Docker Autoscaler (#1221)

## Description

Remove `min_size` and `max_size` from `ignore_changes` from the ASG for
Docker Autoscaler. The reasoning described in the comment was poor;
min_size is fixed at zero, so won't change, and max_size should change
when runner_worker.max_jobs is changed, and will not cause immediate
scale-up. If it causes scale-down, this the logical desired behaviour.

---------

Co-authored-by: Matthias Kay <[email protected]>

Author: jonmcewen

docker_autoscaler.tf

@@ -143,11 +143,9 @@ resource "aws_autoscaling_group" "autoscaler" {
   }
 
   lifecycle {
-    # do not change these values as we would immediately scale up/down, which is not wanted
+    # do not change desired_capacity as this is controlled at runtime by the runner autoscaler
     ignore_changes = [
-      desired_capacity,
-      min_size,
-      max_size
+      desired_capacity
     ]
   }
 }