Fix docker parent image by its digest (#2507)
Fix docker parent image by its digest (#2507)

Previously we just used a tag. Tags a mutable and don't describe the actual image. The image described by a tag could be swapped out on the registry. Using the digest means we use CaS and ask for the specific image we expect.
project-oak/oak,project-oak/oak,project-oak/oak,project-oak/oak,project-oak/oak,project-oak/oak,project-oak/oak