A git ref accepts characters that a Docker tag does not, such as "/" in
feature/foo, which made the promote and retag steps fail. Tag images with
a slug of the ref instead (anything outside [A-Za-z0-9._-] becomes "-",
at most 100 chars), as payments already does. develop, staging and
release tags are unchanged.
AI-assisted-by: Claude
Signed-off-by: David Barragán Merino <david.barragan@kaleidos.net>