Defined in github.com/aws/aws-sdk-go/service/ecs/api.go
If the essential parameter of a container is marked as true, and that container fails or stops for any reason, all other containers that are part of the task are stopped. If the essential parameter of a container is marked as false, then its failure does not affect the rest of the containers in a task. If this parameter is omitted, a container is assumed to be essential.
All tasks must have at least one essential container. If you have an application that is composed of multiple containers, you should group containers that are used for a common purpose into components, and separate the different components into multiple task definitions. For more information, see Application Architecture (http://docs.aws.amazon.com/AmazonECS/latest/developerguide/application_architecture.html) in the Amazon EC2 Container Service Developer Guide.