ECS CLI v1.11.0 was just released, however, it has a bug due to the IpcMode and PidMode feature. This error is shown if the fields are not used:

ERRO[0000] Error registering task definition             error="ClientException:  is not a valid value for pidMode. The value must be one of [task,host]\n\tstatus code: 400

We are actively working to fix this issue. We do not recommend downloading ECS CLI v1.11.0.