github action to build and push django-docker-box to dockerhub

[mithron]

Hello!

This is github action to checkout that repo, checkout django repo and make django-docker-box which can be used to run tests. It DOES NOT run django tests, it's purpose is to provide utility for running django tests in docker. It needs repo owner to define repo secrets for dockerhub: DOCKERHUB_USERNAME and DOCKERHUB_TOKEN to be able to login to dockerhub and push/pull images.

The resulting image example is https://hub.docker.com/repository/docker/mithyronnie/django-docker-box .

P.S. it's done in djangocon sprint so it may be not perfect and ofc comments are appreciated. Thanks in advance. BR Ivan.