Add base-devel tag

This does a few things (and sorry, I know this is quite a lot and I usually try to keep it small but the refactoring kinda needs everything to go in at once):

• Change from image format from archlinux/base to archlinux/archlinux:base
• Add base-devel tag
• Try to supersede !36 (closed)
• Try to address concerns in #35 (closed)
• Set up some automatic publishing preparation
• Add container security scanning because why not

Fixes #35 (closed).

Once this is merged, it needs some manual outside actions:

• Create new repository for the Docker Hub organisation: archlinux/archlinux
• Create a Docker Hub technical account and add it to the organisation
• Delete the old archlinux/base on Dockerhub (as we use archlinux/archlinux with tags now)
• Clean up references to add-base-devel-tags
• Delete old testing garbage (tags, files, releases)

Also for this change:

• Make sure that the security concept is properly observed: Do not let an image or artifact built on non-secure runners pass over to secure runners! Use the non-secure runners for testing only and use the secure runners for final artifact creation and such.