Add variable to push "latest" tag

Repositories often offer an image tagged latest that refers to the latest stable version of the application. It would be nice if the Container Image templates did this automatically.

I propose adding a boolean variable PUSH_IMAGE_LATEST_TAG to docker-image.gitlab-ci.yml and kaniko-image.gitlab-ci.yml. When set to true, a procedure would run that would select the latest stable tag of the image and push that as latest.