Add docker support

[maddouri]

Hello,

This is a followup to #85 I think that it may prove interesting to have a docker build. Most notably, it allows the following:

1. Controlled and consistent builds across a variety of platforms,
2. Getting started easily and quickly with Cling. (cf. tools/docker/README.md)

I wasn't sure where exactly to put the files, so I used tools/docker. Feel free to propose another location.

I have already published a standalone version at https://github.com/maddouri/cling-ubuntu-docker You can test it by running the following:

# get the build from docker hub
docker pull maddouri/cling-ubuntu-docker
# run it (the entry point is cling)
docker run -it maddouri/cling-ubuntu-docker