Build docker images automatically

[JayVii]

This PR creates a CD workflow that will build a new docker-image for every push to a tag (including the creation of a tag) in the form of v1.2.3.

The images will be pushed to the github registry of this repository with the image tags taken from the git tag (i.e. for the major 1, the major-minor 1.2, the major-minor-patch 1.2.3 as well as the latest tag.

The Dockerfile will by default build from the main branch, which makes development easier in a docker-workflow. The CD however will always use the tag as build-arg.

This is my first github-workflow (I did write many gitlab CI/CD pipelines). Information was taken from https://github.com/docker/build-push-action & https://github.com/docker/metadata-action

I did test this CD in my own fork: https://github.com/JayVii/ktistec/actions & https://github.com/JayVii/ktistec/tree/test_actions

I did testing builds (some with fake versions and some with proper) that have been pushed here: https://github.com/JayVii/ktistec/pkgs/container/ktistec

[JayVii]

@toddsundsted you did mention, that you would like to see this for ktistec, so I thought I might try to create it. If this gets merged, I can finally get rid of my manual builds

[JayVii]

This is different from #47 in that it uses Github's registry rather than dockerhub and uses tagging. #47 uses docker/setup-buildx-action, which if I am not mistaken can cross-build, which would be a nice addition here too

[toddsundsted]

Sorry for the super-slow review. One question on build step version.