CNG fails to tag for auto-deploys

Summary

When auto-deploy sends a tag to CNG for building, it occasionally will not tag the image and will only push latest instead. This is problematic as it is expected we can utilize the tagged built image for upcoming auto-deploys. This appear to only impact builds where there are no changes.

Example build where a docker build was tagged and pushed to the registry successfully:

• https://dev.gitlab.org/gitlab/charts/components/images/-/jobs/7016636

Example build where a docker build was not tagged, and only latest was pushed:

• https://dev.gitlab.org/gitlab/charts/components/images/-/jobs/7023692

Potential root cause is an empty GITLAB_REF_SLUG: https://gitlab.com/gitlab-org/build/CNG/blob/master/build-scripts/build.sh#L160