fix: Don't look up latest task definition by default (closes #72)

A data lookup for the latest task definition revision to be used in the
ECS service was enabled by default, which continually appeared in
plans/applies.

This is used when the task definition's updated by an external
deployment tool, so this is now disabled and requires enabling the
`external_task_definition_updates` variable so the default behaviour is
quiet.

main.tf

@@ -77,6 +77,9 @@ locals {
     },
   ]
 
+  # ECS task definition
+  latest_task_definition_rev = var.external_task_definition_updates ? max(aws_ecs_task_definition.atlantis.revision, data.aws_ecs_task_definition.atlantis[0].revision) : aws_ecs_task_definition.atlantis.revision
+
   # Secret access tokens
   container_definition_secrets_1 = local.secret_name_key != "" && local.secret_name_value_from != "" ? [
     {
@@ -563,6 +566,8 @@ resource "aws_ecs_task_definition" "atlantis" {
 }
 
 data "aws_ecs_task_definition" "atlantis" {
+  count = var.external_task_definition_updates ? 1 : 0
+
   task_definition = var.name
 
   depends_on = [aws_ecs_task_definition.atlantis]
@@ -571,10 +576,8 @@ data "aws_ecs_task_definition" "atlantis" {
 resource "aws_ecs_service" "atlantis" {
   name    = var.name
   cluster = module.ecs.this_ecs_cluster_id
-  task_definition = "${data.aws_ecs_task_definition.atlantis.family}:${max(
-    aws_ecs_task_definition.atlantis.revision,
-    data.aws_ecs_task_definition.atlantis.revision,
-  )}"
+
+  task_definition                    = "${var.name}:${local.latest_task_definition_rev}"
   desired_count                      = var.ecs_service_desired_count
   launch_type                        = "FARGATE"
   deployment_maximum_percent         = var.ecs_service_deployment_maximum_percent

variables.tf

@@ -381,6 +381,12 @@ variable "ulimits" {
   default = null
 }
 
+variable "external_task_definition_updates" {
+  description = "Enable to allow the task definition to be updated outside of this Terraform module. This should be enabled when using a deployment tool such as ecs-deploy which updates the task definition and will then keep the ECS service using the latest version of the task definition."
+  type        = bool
+  default     = false
+}
+
 # https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_FirelensConfiguration.html
 variable "firelens_configuration" {
   description = "The FireLens configuration for the container. This is used to specify and configure a log router for container logs. For more details, see https://docs.aws.amazon.com/AmazonECS/latest/APIReference/API_FirelensConfiguration.html"