When development of salsa-ci-pipeline is done on a branch, the images
are tagged according to `CI_COMMIT_REF_NAME`. However the set of
characters allowed for a git ref name is wider than the set of
characters allowed for a image tag, so some branch names fail the
pipeline. For example, with a branch named `wip/apt-details`:

```
$ buildah build [...]
Error: tag base:sid_wip/apt-details: invalid reference format
```

The `/` is a commonly-used character in Git branch names (I personally
use it all the time), but it's not the only one, eg. `+` or `@` are
allowed in git branch names, but will fail the pipeline all the same.

It looks like the set of characters allowed for a image tag is defined
at <https://docs.docker.com/reference/cli/docker/image/tag/>:

> The tag must be valid ASCII and can contain lowercase and uppercase
> letters, digits, underscores, periods, and hyphens.

It turns out that GitLab already has a pre-defined variable that fits
the bill, named `CI_COMMIT_REF_SLUG`, defined as such:

> `CI_COMMIT_REF_NAME` in lowercase, shortened to 63 bytes, and with >
> everything except `0-9` and `a-z` replaced with `-`. No leading /
> trailing `-`.  Use in URLs, host names and domain names.

IMO, better make use of `CI_COMMIT_REF_SLUG` and make it easier for
contributors.