How to run a single ECS task in only one EC2 instance using auto scaling and capacity provider?

Question

I am using Terraform to create an ECS cluster with an EC2 instance. My goal is to have a single task running on only one EC2 instance. I am managing both the capacity provider and auto-scaling for this cluster.

Initially, the deployment of a task to an EC2 instance runs smoothly. However, when I try to deploy a new task definition to replace the existing task, the ECS keeps the task in a PROVISIONING state. The task stays in this state until I change the auto-scaling group's max_size from 1 to 2. Once I do that, the new task deployment is done in a new EC2 instance, and the previous instance is removed after some time as a action related to a CloudWatch alarm (CapacityProviderReservation < 100 for 15 datapoints within 15 minutes).

For now, in my non-production environment, I want to keep only one instance as part of my cluster and allow multiple deployments of the same task on the same instance.

Terraform code:

# ECS service
resource "aws_ecs_service" "this" {
  name                 = "cluster"
  iam_role             = aws_iam_role.ecs_role.arn
  cluster              = aws_ecs_cluster.cluster.id
  task_definition      = aws_ecs_task_definition.task_definition.arn
  desired_count        = 1
  force_new_deployment = true

  load_balancer {
    target_group_arn = aws_alb_target_group.lb.arn
    container_name   = aws_ecs_task_definition.task_definition.family
    container_port   = 80
  }

  ordered_placement_strategy {
    type  = "binpack"
    field = "memory"
  }

  capacity_provider_strategy {
    capacity_provider = aws_ecs_capacity_provider.ecs_capacity_provider.name
    base              = 1
    weight            = 100
  }

  lifecycle {
    create_before_destroy = true
    ignore_changes = [
      desired_count
    ]
  }

}

# Auto scaling
resource "aws_autoscaling_group" "ecs_asg" {
  name                  = "asg"
  vpc_zone_identifier   = [for subnet in var.public_subnet_ids : subnet]
  max_size              = 1
  min_size              = 1
  desired_capacity      = 1
  health_check_type     = "EC2"
  protect_from_scale_in = false

  launch_template {
    id      = aws_launch_template.template.id
    version = "$Latest"
  }

  instance_refresh {
    strategy = "Rolling"
  }

  lifecycle {
    create_before_destroy = true
  }

}

# capacity provider
resource "aws_ecs_capacity_provider" "ecs_capacity_provider" {
  name = "ecs_capacity_provider"

  auto_scaling_group_provider {
    auto_scaling_group_arn         = aws_autoscaling_group.ecs_asg.arn
    managed_termination_protection = "DISABLED"

    managed_scaling {
      maximum_scaling_step_size = 2
      minimum_scaling_step_size = 1
      status                    = "ENABLED"
      target_capacity           = 100
    }
  }

}

resource "aws_ecs_cluster_capacity_providers" "ecs_capacity_providers" {
  cluster_name = aws_ecs_cluster.cluster.name

  capacity_providers = [aws_ecs_capacity_provider.ecs_capacity_provider.name]

  default_capacity_provider_strategy {
    base              = 1
    weight            = 100
    capacity_provider = aws_ecs_capacity_provider.ecs_capacity_provider.name
  }

}

# ECS Task
resource "aws_ecs_task_definition" "task" {
  family = "task"
  container_definitions = jsonencode([
    {
      name      = "task",
      image     = "test",
      cpu       = "768",
      memory    = "4096",
      essential = true
      portMappings = [
        {
          containerPort = 80
          hostPort      = 80
          protocol      = "tcp"
        }
      ]
      logConfiguration = {
        logDriver = "awslogs",
        options = {
          "awslogs-group"         = aws_cloudwatch_log_group.logs.name,
          "awslogs-region"        = var.region,
          "awslogs-stream-prefix" = "app"
        }
      }
    }
  ])
  execution_role_arn = aws_iam_role.ecs_exec.arn
  task_role_arn      = aws_iam_role.ecs_task.arn

}

I noticed there is a related cloud watch alarm being triggered when I try the second task deployment which is linked to the auto scaling:

Alarm: "TargetTracking-test-ecs-asg-AlarmHigh-e5a4556-5686-5669-26546-e745a5ed90cb"