Building mageia cauldron images

Up until now we only had mageia docker images available for download at docker hub of stable releases, but since this week we have cauldron (the development version of mageia) images too, also for x86_64, armv7hl and aarch64 architectures. I had not updated the mageia docker images in a while because of lack of time, even thougth we now have an automated process for building, pushing and releasing updated versions of them. After some small fixes to the build2release program it was rather easy to add the missing cauldron images to the build list, along with mageia 7.

The process the build2release program automates is roughly something like this:

1. Checkout docker-brew-mageia repository
2. Delete dist branch and recreate it from master branch. This is is done so this branch only contains one commit with the latest os root tarball files and avoid having a commit history wasting a lot of space.
3. Checkout the new dist branch.
4. Build the root tarball for all supported versions and architectures.
5. Commit the new tarballs in dist branch and force push it.
6. Update my fork of docker-library with latest upstream changes.
7. Update the mageia library file with the new commits, tags and images.
8. Commit the changes to the mageia library file and create a PR to get them merged into upstream.
9. Wait for the PR to be reviewed, accepted and merged so new images are built and published at docker hub.

The program still needs some work on the last steps from 7 onwards. Updating the docker-library fork works but is missing to be able to add new images to it, for now its only able to update existing ones, and the PR creation is still experimental. So for now some work is still manually done to push updated images to docker hub. I have pushed updated mageia 7 images and the first build of cauldron images, available as mageia:cauldron.