When transforming branch name into image label while running CI checks, take care not to create invalid label

[jiridanek]

Error: tag ghcr.io/red-hat-data-services/notebooks/workbench-images:base-ubi8-python-3.8-renovate/registry.access.redhat.com-ubi9-ubi-minimal-9.x_fcb4541c34b36a9781c20d9164d58d9e6a57a097: invalid reference format

https://github.com/red-hat-data-services/notebooks/actions/runs/10055394214/job/27791966809?pr=287#step:9:15