Description:
Docker added a new switch --init to the run command which starts a pseudo-init daemon before the command.
This daemon is there to reap all zombies that the container could create.
For this, the package in docker 1.13 started to ship a init replacement base on tini (https://github.com/krallin/tini) which is named docker-init.

Additional info:
docker-init started shipping in version 1.13.
Documentation will be added in the upcoming release v17.03.0.
See merge request https://github.com/docker/docker/pull/30523

Steps to reproduce:
docker run --init busybox /bin/bash
Fails with :
docker: Error response from daemon: exec: "docker-init": executable file not found in $PATH.
ERRO[0003] error getting events from daemon: net/http: request canceled