When there is no AWS_CONTAINER_CREDENTIALS_RELATIVE_URI variable set to the container, aws cli and tools based on official SDKs will not pick up task role and continue using ec2 cluster instances role instead.

For reference:

https://github.com/aws/aws-sdk-ruby/issues/1309

http://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-iam-roles.html