How should the DevOps engineer update the CloudFormation template to resolve this issue?

Explanations:

The AWS::ECS::Cluster resource does not have a direct reference to EC2 instances. Instead, EC2 instances are registered with the ECS cluster through their configuration in the Auto Scaling group and the UserData script, not by referencing them in the cluster resource.

Referencing the ECS cluster in the UserData property of the AWS::AutoScaling::LaunchConfiguration ensures that the EC2 instances are correctly configured to register with the specified ECS cluster upon launch. This is typically done using theECS_CLUSTERenvironment variable in the UserData script.

The AWS::EC2::Instance resource is not the correct resource to reference for ECS cluster registration. Instances need to be configured through the Auto Scaling launch configuration or template, specifically in the UserData property, rather than attempting to directly reference ECS in the EC2 instance settings.

Using a Custom Resource to trigger an AWS Lambda function for instance registration adds unnecessary complexity. While it could theoretically work, it is not the standard or simplest approach to ensure EC2 instances are registered with the ECS cluster, especially since this can be achieved directly in the launch configuration’s UserData.