Add tini to container

[felipecrs]

While the README suggests to use docker run --init, the bullet-proof approach would be to bundling tini in the container itself, as stated in the docker best practices guides for the official images:

https://github.com/docker-library/official-images#init

Its binary is extremely tiny and should not cause any noticeable increase in the size of the docker image.

[wperron]

Very much in favor, I actually just hit the issue of not handling SIGTERM on a demo Docker container, --init is super easy to forget or simply not know about.