You can deploy the Datadog Agent as a Convox app by using a docker-compose.yml manifest. Use a count that matches the InstanceCount parameter of your Rack.
# check out the repo$ git clone https://github.com/convox-examples/datadog.git
$ cd dd-agent
# deploy the agent app and secret$ convox apps create
$ convox env setDD_API_KEY=<your api key>
$ convox deploy
$ convox scale agent --count=3 --cpu=10 --memory=128
Run convox deploy to deploy Datadog Agent into ECS.