Distribution as a docker image as well

[IreshMM]

This is a proposal to have a Dockerfile in the repository for an official container image for those who are looking to use jinja2-cli in a container. Of course, we can always have our own image created having jinja2-cli installed in it but I suppose having an official image with a well defined, easy to use interface for passing inputs and outputs to/from the container, to use it would be favorable for most people.

TL/DR: Adding a new release channel as a docker image.