Fix build scripts to better support upstream triggers

What does this MR do?

• Fix check for image existence
• Use correct upstream data for tagging
• Add ability to override CACHE_BUSTER

This is the first change related to Downstream CNG pipeline build time improvements (gitlab-org/quality/quality-engineering&89 - closed) which was implemented and tested via quality specific CNG fork. This and all upcoming changes have been running in gitlab-org/gitlab scheduled master pipelines and have been executing full e2e test suite against produced images.

Related issues

• gitlab-org/quality/quality-engineering&89 (closed)
• gitlab-org/quality/quality-engineering/team-tasks#2841 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated
• New dependencies are managed with dependencies.io