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

Error: unable to parse value provided [ghcr.io/RomanFilip/notebooks-fork/workbench-images/build-cache] to --cache-to: invalid repo "ghcr.io/RomanFilip/notebooks-fork/workbench-images/build-cache": must contain registry and repository: invalid reference format: repository name must be lowercase

https://github.com/RomanFilip/notebooks-fork/actions/runs/13499383840/job/37713825404#step:17:95

[jiridanek]

      - name: downcase REPO
        run: |
          echo "REPO=${GITHUB_REPOSITORY,,}" >>${GITHUB_ENV}

https://github.com/orgs/community/discussions/25768

[jiridanek]

      - name: downcase IMAGE_REGISTRY and CACHE
        run: |
          echo "IMAGE_REGISTRY=${IMAGE_REGISTRY,,}" >>${GITHUB_ENV}
          echo "CACHE=${CACHE,,}" >>${GITHUB_ENV}

[jiridanek]

not completely fixed, we did not address the "branch name too long" source of problems