gitlab-ci: sanitize git ref names for use as docker tags
git ref names can contain many chars that are not allowed in docker tags.
https://docs.docker.com/engine/reference/commandline/tag/ A tag name must be valid ASCII and may contain lowercase and uppercase letters, digits, underscores, periods and dashes. A tag name may not start with a period or a dash and may contain a maximum of 128 characters.
https://gitlab.com/brandsimon/fdroidserver/-/jobs/3705004274