gitlab-ci: Fix docker image ref after move to Kaniko

After 5ef5bacf "Build the docker image with Kaniko" we started to upload images with the commit sha in the image name rather than using it as a tag ($CI_REGISTRY_IMAGE/runtime/$CI_COMMIT_SHA instead of $CI_REGISTRY_IMAGE/runtime:$CI_COMMIT_SHA).

This causes the tagging of the latest image on the main branch to fail with:

$ skopeo copy --src-creds "$AUTH" --dest-creds "$AUTH" docker://"${IMAGE}:${TAG_OLD}" docker://"${IMAGE}:${TAG_NEW}"
time="2023-02-24T08:07:27Z" level=fatal msg="Error initializing source
docker://registry.gitlab.apertis.org/infrastructure/qa-report-app/runtime:03b1892be15e4ed0743104b13dfa69f86e2b7c9a:
Error reading manifest 03b1892be15e4ed0743104b13dfa69f86e2b7c9a in
registry.gitlab.apertis.org/infrastructure/qa-report-app/runtime: manifest unknown: manifest unknown"