Wrap Node.js server with tini in Docker image

[dtan4]

We cannot stop elasticsearch-head Docker container gracefully, because Node.js in Docker container cannot handle kernel signal (SIGINT, SIGTERM and so on).
https://github.com/nodejs/docker-node/blob/078ff84cb756df33fe3444090914e65a0721c9fd/docs/BestPractices.md#handling-kernel-signals

To handle these signals, we have to wrap Node.js process with lightweight init system, for example tini, or use docker run --init flag.
However, Kubernetes does not support this flag.

I added some steps to install tini in elasticsearch-head Docker image. All Docker users will be able to stop gracefully by this change.

[rejinka]

Did you take a look at #362? I think this should also solve your problem with the kernel signals. As a plus, it shrinks the image to 2% of its original size :-)