We try to re-use as much as possible the docker compose (v3) reference as much as possible.
For the definition of the services, you can simply use the already existing Docker compose definition for your services. However, there are only a limited number of settings that are today working:
Any property in the docker-compose file you have today, for example, build is simply ignored. It will be neither removed nor modified
Checkout the ECS ComposeX secrets definition syntax secrets to import AWS Secrets Manager secrets to your container.