Create a Gitpod workspace image

What does this Merge Request do and why?

Improve gitpod workspace image build

This is a major change to how we build the image, instead of basing our workspace image on gitpod/workspace-full we are now utilizing the much smaller gitpod/workspace-base which doesn't ship things like e.g. java.

Furthermore we are now utilizing asdf and our make bootstrap installation. This reduces the image size immensely, hopefully leading to faster startup times. We went down from 5.7 GB => 2.9 GB.

Additionally we are preparing the CI pipeline for scheduled builds so that we can update the gitpod workspace image more often.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed Follow-up in: #1108 (closed)
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.