Tag asset docker image with GitLab version also

Balasankar 'Balu' Crequested to merge
tag-pipeline-asset-image into master

What does this MR do?

In additino to CI_COMMIT_REF_SLUG, tag the docker image containing assets with the GitLab version also, if the pipeline is a tag pipeline. This will help other components of the release to identify what tag of the image they should wait for, without needing to compute the slug.

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