Add example for usage with docker container

[x0ddf]

FIY @khos2ow

Preface.

Terraform-docs repository has the ability to run it from the docker container. But example how to build and pack a custom plugin(formatter) for the docker container is missing somehow.

Proposal

1. add to the make ability to build/push images
2. add Dockerfile
3. add usage example to the readme.