[CNG] [CI] Fix UTC midnight crossover failures

Currently the CNG pipeline uses several instances of date -u +%D that causes problems when jobs run on either side of UTC midnight. This is due to the date -u +%D is used as a component in the generation of the image tag of several of the intermediate images. These occurrences should be replaced with CI_PIPELINE_CREATED_AT CI variable so that the hash has consistent value irrespective to when the job runs.