Add improved Dockerfile with wrapper script for seamless upgrades

This Dockerfile builds from the local Twister repository checkout. That means it can work as a [trusted build](https://index.docker.io/help/docs/#trustedbuilds), so every new commit on the repository will trigger an automatic build of a new docker image on the [docker index](https://index.docker.io/).

I already set up a trusted build for my fork of twister-core. As soon as this PR is merged, @miguelfreitas could register on the docker index and create a trusted build for the master branch on the main repository (miguelfreitas/twister-core). So there will always automatically be a ready-to-run docker image for the current twister HEAD on the docker index.

Additionally, this image uses a docker volume for storing twister's data in `~./twister`. Using the included wrapper script, this makes it easy to upgrade to a new master, while keeping the data:

    git pull
    ./docker-container stop
    ./docker-container build
    ./docker-container run

(Note: This can still be improved to prevent twister downtime during build.)

This could solve #201 and #195, at least for Linux users (as docker is still not that easy to run on Windows/Mac).