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Current implementation was using `$CI_COMMIT_SHA` to set the development version of the image. It's not the best solution, because the cleanup job (executed on environment stop => MR merging) will clean-up only the image built for the latest commit from latest MR update. By using `$CI_MERGE_REQUEST_IID` we make the image to follow the MR IID (which is unchanged during development), so the cleanup job should be able to remove all unneeded images.
