Build example:
packer build \
-var 'ddagent_api_key=<your_dd_api_key>' \
-var 'ami_id=<the_ecs_ami_base_from_devguide>' \
-var 'aws_region=us-west-2' \
packer-ddagent.json
You can select the AMI Base Id from ECS Optimized-AMI list. Just select the AMI Base from the AWS-Region your infrastructure are in.