Build docker image based on asdf

What does this Merge Request do?

The problem: We want to rebuild the image, if the Dockerfile, .tool-versions or packages.txt change. Unfortunately that isn't possible to do that in GitLab CI, because we cannot set a variable based on the contents of files. That is why we use this algorithm:

Check if a docker image for our branch exists, e.g. base_name/branch-name:latest. If it exists, we pull it and compare the dependencySHA label on it to the shasum of the Dockerfile, packages.txt and .tools-versions. If the label matches the shasum, we do not build the image.

Otherwise, we check if a caching image, e.g. base_name/asdf-cache:$dependencySHA exists. If it exists, we pull it and push it under the base_name/branch-name:latest. This operation is rather space-efficient, as the existing layers will be re-used. It also allows us to re-use images across branches if the dependencies didn't change.

If none of the above allows us to use a cached image, we will build one.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.