The ECS instance fails to join the ECS cluster because it relies on docker to launch its ecs agent. If in this case docker fails to start then the whole process halts.
Docker fails to start in AWS ECS instances with the following message in /var/log/docker :
Error starting daemon: error initializing graphdriver: devicemapper: Non existing device docker-docker--pool
Notice the following.
Instance AMI is a COPY of the AWS ECS optimized AMI with the embedded ECS agent(and docker config for ECS)