this datasource allows terraform to work with externally modified state, e.g.
when you're using an ECS service which is continously updated by your CI via the
AWS CLI.
right now you'd have to wrap terraform into a shell script which looks up the
current image digest, so running terraform won't change the updated service.
using the aws_ecs_container_definition data source you can now leverage
terraform, removing the wrapper entirely.
stack72