Add basic docker container image build support

[bpkroth]

This adds very basic support to build the current tree into a docker container image.

Alternatively, one can use the pip published package to install into the container, though this version is probably more amenable to supporting both published and local development versions more easily.

An improved version would also publish the resulting image to mcr/dockerhub.

One can build the image with

make

and run it with

docker run -it --rm mssql-cli --help