tini is included in Docker since Docker 1.13

[Tarnasa]

According to https://github.com/krallin/tini#using-tini

NOTE: If you are using Docker 1.13 or greater, Tini is included in Docker itself. This includes all versions of Docker CE. To enable Tini, just pass the --init flag to docker run.

So there's no reason for adding them to docker containers ourselves, the arena just needs to pass the --init flag.