Docker build and push to github container registry

[floriandulzky]

Expected behavior

A github action that creates a docker image and push it automatically to the github container registry (ghcr.io) See fork: https://github.com/dradis/dradis-ce/compare/develop...floriandulzky:dradis-ce:feature/docker-build

Did not create a PR yet. Will update the documentation and then make the PR. But I want to make sure this "feature" is in your interest?