It also needs to reap orphan processes or they will become zombies. The dumb-init code does not appear to be doing that so I reported an issue[1].
In general docker is half-trying to be the init system, but most people using it are putting a whole child os with its own init system in their container. I think the approach that rkt uses where it uses systemd to run the process is safer. Now if people would just start using lightweight containers...