ECS provisions multiple servers but never runs the task - amazon-ec2

Question

I have an ECS cluster where the capacity provider is an auto-scaling group of ec2 servers with a Target Tracking scaling policy and Managed Scaling turned on.
The min capacity of the cluster is 0, the max is 100. The instance types it's employing are c5.12xlarge.
I have a task that uses 4 x vCPUs and 4 GiB memory. When I run a single instance of that task on that cluster, ECS very slowly auto scales the group to > 1 servers (usually 2 to begin with, and then eventually adds a third one - I've tried multiple times), but never actually runs the task and stays in a state of PROVISIONING for ages and ages before I get annoyed and stop the task.
Here is a redacted copy of my task description:
{
"family": "my-task",
"taskRoleArn": "arn:aws:iam::999999999999:role/My-IAM-Role",
"executionRoleArn": "arn:aws:iam::999999999999:role/ecsTaskExecutionRole",
"cpu": "4 vCPU",
"memory": 4096,
"containerDefinitions": [
{
"name": "my-task",
"image": "999999999999.dkr.ecr.us-east-1.amazonaws.com/my-container:latest",
"essential": true,
"portMappings": [
{
"containerPort": 12012,
"hostPort": 12012,
"protocol": "tcp"
}
],
"mountPoints": [
{
"sourceVolume": "myEfsVolume",
"containerPath": "/mnt/efs",
"readOnly": false
}
]
}
],
"volumes": [
{
"name": "myEfsVolume",
"efsVolumeConfiguration": {
"fileSystemId": "fs-1234567",
"transitEncryption": "ENABLED",
"authorizationConfig": {
"iam": "ENABLED"
}
}
}
],
"requiresCompatibilities": [
"EC2"
],
"tags": [
...
]
}
My questions are:
Why, if I'm running a single task that would easily run on once instance, is it scaling the group to at least 2 servers?
Why does it never just deploy and run my task?
Where can I look to see what the hell is going on with it (logs, etc)?

Answer 1

So it turns out that, even if you set an ASG to be the capacity provider for an ECS cluster, if you haven't set the User Data up in the launch configuration for that ASG to have something like the following:
#!/bin/bash
echo ECS_CLUSTER=my-cluster-name >> /etc/ecs/ecs.config;echo ECS_BACKEND_HOST= >> /etc/ecs/ecs.config;
then it will never make a single instance available to your cluster. ECS will respond by continuing to increase the desired capacity of the ASG.
Personally I feel like this is something that ECS should ensure happens without your knowledge. Maybe there's a good reason why not.