gitlab: stable staging branches publish containers in a separate tag

gitlab: stable staging branches publish containers in a separate tag

If the stable staging branches publish containers under the 'latest' tag
they will clash with containers published on the primary staging branch,
as well  as with each other. This introduces logic that overrides the
container tag when jobs run against the stable staging branches.

The CI_COMMIT_REF_SLUG variable we use expands to the git branch name,
but with most special characters removed, such that it is valid as a
docker tag name. eg 'staging-8.0' will get a slug of 'staging-8-0'

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230608164018.2520330-4-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>