Update auto-build-image to a version with CNB support and add docs

Once gitlab-org/cluster-integration/auto-build-image!30 (merged) is merged, a new version of auto-build-image will be released with opt-in support for Cloud Native Buildpacks. As a follow up, we need to

• Update the build template
• Update the docs to mention this new feature

See also #25954 (closed)