A task definition for ECS using MeltWater's Docker Container and Image clean-up containerized script. Please see https://github.com/meltwater/docker-cleanup for more details on correct usage.

DockerCleanup-Task.json

{
    "containerDefinitions": [
        {
            "volumesFrom": null,
            "memory": 32,
            "extraHosts": null,
            "dnsServers": null,
            "disableNetworking": true,
            "dnsSearchDomains": null,
            "portMappings": [],
            "hostname": null,
            "essential": true,
            "entryPoint": null,
            "mountPoints": [
                {
                    "containerPath": "/var/run/docker.sock",
                    "sourceVolume": "docker_socket",
                    "readOnly": null
                },
                {
                    "containerPath": "/var/lib/docker",
                    "sourceVolume": "docker_data",
                    "readOnly": null
                }
            ],
            "name": "DockerCleanup-Container",
            "ulimits": null,
            "dockerSecurityOptions": null,
            "environment": [
                {
                    "name": "CLEAN_PERIOD",
                    "value": "**None**"
                },
                {
                    "name": "DELAY_TIME",
                    "value": "**None**"
                },
                {
                    "name": "KEEP_IMAGES",
                    "value": "**None**"
                },
                {
                    "name": "KEEP_CONTAINERS",
                    "value": "**None**"
                },
                {
                    "name": "KEEP_CONTAINERS_NAMED",
                    "value": "**None**"
                },
                {
                    "name": "LOOP",
                    "value": "true"
                },
                {
                    "name": "DEBUG",
                    "value": 0
                }
            ],
            "links": null,
            "workingDirectory": null,
            "readonlyRootFilesystem": true,
            "image": "meltwater/docker-cleanup:1.7.1",
            "command": null,
            "user": null,
            "dockerLabels": null,
            "logConfiguration": null,
            "privileged": null
        }
    ],
    "volumes": [
        {
            "host": {
                "sourcePath": "/var/run/docker.sock"
            },
            "name": "docker_socket"
        },
        {
            "host": {
                "sourcePath": "/var/lib/docker"
            },
            "name": "docker_data"
        }
    ],
    "family": "DockerCleanup-Task"
}

@ernestm, a mild hack could be to expose a port that you're not using anywhere else. This would force all containers to be scheduled on distinct EC2 Instances.