Update image ID in launch template with instance refresh

* Modifies the Lambda that triggers an instance refresh, to first update
  the image ID to the latest AMI. This ensures the instances will be
  using the most up to date AMI
* This has been placed within the instance refresh lambda, because it
  will require an instance refresh anyway to actually update the
  instances, and allows it to happen at the chosen instance refresh
  cron
* The AutoScaling group's launch template version has also been changed
  to '$Latest', rather than a specific version so that we don't need to
  keep that up to date with the latest version

ecs-cluster-infrastructure-instance-refresh-lambda.tf

@@ -98,7 +98,9 @@ resource "aws_lambda_function" "ecs_cluster_infrastructure_instance_refresh" {
 
   environment {
     variables = {
-      asgName = aws_autoscaling_group.infrastructure_ecs_cluster[0].name
+      asgName            = aws_autoscaling_group.infrastructure_ecs_cluster[0].name
+      launchTemplateName = aws_launch_template.infrastructure_ecs_cluster[0].name
+      amiVersion         = local.infrastructure_ecs_cluster_ami_version
     }
   }
 

ecs-cluster-infrastructure.tf

@@ -176,7 +176,7 @@ resource "aws_autoscaling_group" "infrastructure_ecs_cluster" {
 
   launch_template {
     id      = aws_launch_template.infrastructure_ecs_cluster[0].id
-    version = aws_launch_template.infrastructure_ecs_cluster[0].latest_version
+    version = "$Latest"
   }
 
   vpc_zone_identifier = local.infrastructure_ecs_cluster_publicly_avaialble ? [

lambdas/ecs-asg-instance-refresh/function.py

@@ -3,9 +3,47 @@
 import os
 
 asgName = os.environ['asgName']
+launchTemplateName = os.environ['launchTemplateName']
+amiVersion = os.environ['amiVersion']
 
 def lambda_handler(event, context):
     asgClient = boto3.client('autoscaling')
+    ec2Client = boto3.client('ec2')
+
+    # Update launch template to use the latest AMI
+    response = ec2Client.describe_images(
+        Owners=['amazon'],
+        Filters=[
+            {'Name': 'name', 'Values': [amiVersion]},
+            {'Name': 'state', 'Values': ['available']},
+            {'Name': 'architecture', 'Values': ['x86_64']}
+        ]
+    )
+
+    images = sorted(
+        response['Images'],
+        key=lambda x: x['CreationDate'],
+        reverse=True
+    )
+    if not images:
+        raise Exception("No AMIs found!")
+
+    latest_ami_id = images[0]['ImageId']
+    print(f"Latest ECS-optimized AMI: {latest_ami_id}")
+
+    try:
+        response = ec2Client.modify_launch_template(
+            LaunchTemplateName=launchTemplateName,
+            LaunchTemplateData={
+                'ImageId': latest_ami_id
+            }
+        )
+        print(f"Launch template updated successfully: {response}")
+    except Exception as e:
+        print(f"Error updating launch template: {e}")
+        raise
+
+    # Start instance refresh
     try:
         response = asgClient.start_instance_refresh(
             AutoScalingGroupName=asgName,