Implement XDG Directory for workspace base image

MR: Pending

Description

Follow-up on discussion here, gitlab-build-images!983 (comment 2618725341), we would like to implement XDG Directory, so the user would have post-start script result perisitent on volume.

But we should first wait until the issue for renaming /project to /workspace (issue Add support for container specific postStart ev... (#553096)), and Persist desired kubernetes configuration on workspace creation+ to be rolled out first.

The original MR should have got some implementation in place already for reference, gitlab-build-images!983 (merged)_

Acceptance criteria

• implement XDG directories to use /workspace
• Test behaviour works as expected (for example, mise install in post-start script will presistent)

Implementation plan

NA