Amazon ECS Container Agent

This builds an Amazon ECS Container Agent, with some patches applied for Remind's use.

Patches

1. 1-tty: This patch provides a method to pass down the -t (allocate TTY) and -i (open stdin) flags of docker run, so that Empire can attach to these tasks. (From https://github.com/aws/amazon-ecs-agent/compare/v1.18.0...ejholmes:tty)
2. 2-pids-limit: This patch will automatically remove the nproc ulimit, and replace it with the --pids-limit flag of Docker run (From https://github.com/aws/amazon-ecs-agent/compare/master...ejholmes:pids-limit)
3. 3-fix-netnsformat: This patch fixes the ecscni module so that the ECS agent can configure network namespaces, when not running within a Docker container. (From https://github.com/aws/amazon-ecs-agent/compare/master...ejholmes:fix-netnsformat)

Updating to a new release

1. Bump REVISION in Makefile
2. Run make clean
3. Run make test
4. Run make