Add_docker_metadata needs to be refactored to get containerID of non docker containers.
https://github.com/elastic/beats/blob/ce68c7814e5c34ff417701481e1f65f1d5e100cf/libbeat/processors/add_docker_metadata/add_docker_metadata.go#L257-L264

Lookup CID using process cgroup membership data can be moved to add_process_metadata
https://github.com/elastic/beats/blob/ce68c7814e5c34ff417701481e1f65f1d5e100cf/libbeat/processors/add_docker_metadata/add_docker_metadata.go#L142-L148

@vjsamuel add anything you think I've missed