Added a Dockerfile to compile and build opentxs lib and cli

The Dockerfile in the root of the repo allows anyone to build and run a opentxs in a docker container. The image is built using docker and copies the local source into a running docker container. 

    e.g, git clone https://github.com/grealish/opentxs.git
    cd opentxs
    docker build .
    docker -i -t run <the build image ID> /bin/bash << any command you want to run or opentxs
    otuser@<continaerID>$

I have uploaded already a build image from my fork if you want to see

    docker run -i -t run grealish/opentxs-lib-cli /bin/bash

More to explain in a docker workshop that i'm planning