Draft: Replace MR-specific CI_COMMIT_SHA with default value

What does this MR do?

A customer provided the following feedback internally. Context: Trying to scan containers while avoiding duplicate vulnerability findings.

I struggle to find the correct value for CS_DEFAULT_BRANCH_IMAGE.

• Setting it to $CI_REGISTRY_IMAGE/$CI_DEFAULT_BRANCH:$CI_COMMIT_SHA works, but an image with this image tag is not existing for the Default Branch. The Commit and its reference is exclusively to the branch until it is merged.

My emphasis. The inclusion of CI_COMMIT_SHA seems OK only in the context of MR pipelines, but I've not tested this: https://docs.gitlab.com/ee/user/application_security/index.html#use-security-scanning-tools-with-merge-request-pipelines

• $CI_REGISTRY_IMAGE/$CI_DEFAULT_BRANCH does not work, but would also make sense, as non existing tags does lead to a working widget [in MR with scanning results].

• Setting it to $CI_REGISTRY_IMAGE/$CI_DEFAULT_BRANCH:latest works as well. Its not given as example in the docs, but makes more sense to me, as an image with latest tag should exist for the default branch.

I'm short on time before OoO, so I'd like to quickly suggest discussing their 2 suggestions, starting with the most sensible & smallest.

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