Terraform Docker Injection - You can override the docker config this way - useful for custom BIP / CIDRs

ecs-cluster.tf

...
# Fetch the AWS ECS Optimized Linux AMI. Note that if you've never launched this AMI before, you have to accept the
# terms and conditions on this webpage or the EC2 instances will fail to launch:
# https://aws.amazon.com/marketplace/pp/B00U6QTYI2
data "aws_ami" "ecs" {
  most_recent = true
  owners      = ["amazon"]

  filter {
    name   = "name"
    values = ["amzn-ami-*-amazon-ecs-optimized"]
  }
}

data "template_file" "ecs_config" {
  template = "${file("user_data.sh")}"

  vars {
    cluster_name = "${var.ecs_cluster_name}"
    username     = "${var.username}"
    password     = "${var.password}"
  }
}
...

user_data.sh

#!/bin/bash
echo "ECS_CLUSTER=${cluster_name}" >> /etc/ecs/ecs.config
echo "ECS_ENGINE_AUTH_TYPE=docker" >> /etc/ecs/ecs.config
echo "ECS_ENGINE_AUTH_DATA={\"custom.repo.internal.local:9090\":{\"username\":\"${username}\",\"password\":\"${password}\"}}" >> /etc/ecs/ecs.config
cat <<EOF > /etc/sysconfig/docker
# The max number of open files for the daemon itself, and all
# running containers.  The default value of 1048576 mirrors the value
# used by the systemd service unit.
DAEMON_MAXFILES=1048576

# Additional startup options for the Docker daemon, for example:
# OPTIONS="--ip-forward=true --iptables=true"
# By default we limit the number of open files per container
OPTIONS="--default-ulimit nofile=1024:4096 --bip=192.168.200.1/24"

# How many seconds the sysvinit script waits for the pidfile to appear
# when starting the daemon.
DAEMON_PIDFILE_TIMEOUT=10
EOF

# Restart the agent due to the fact that bootstrapping order is weird!
service docker restart
docker start ecs-agent