Adding a ACM certificate to a ecs task def

ACM certs can be used with selected services like CloudFront, load balancers, API Gateway, and others https://docs.aws.amazon.com/acm/latest/userguide/acm-services.html

ECS isn't on this list. If you want to use a cert in ACM with ECS then consider putting a load balancer in front of ECS https://docs.aws.amazon.com/AmazonECS/latest/userguide/create-application-load-balancer.html