Follow-up from "ci:add static versions for base images"

The following discussion from !20047 (merged) should be addressed:

• @Killian-Delarue started a discussion: (+2 comments)

  Maybe that would be simplier if Base_image had a type t = {image: Image.t; version: version} ?

  Then, a base ifmage would be defined as debian_unstable : t and could be use aither with debian_unstable.image or debian_unstable.version.

  What do you think ?