Switch guide runtime from Chainguard to Google distroless

[beyildirim]

This replaces the guide image runtime base from Chainguard Python with pinned Google distroless Python to avoid the Dependabot failure on chainguard/python and keep the guide runtime on a vendor-neutral distroless base.

Validation:

• Resolved the current multi-arch digest for gcr.io/distroless/python3-debian12:nonroot via docker buildx imagetools inspect.
• Confirmed the image metadata exposes /usr/bin/python3.11 as the entrypoint, so the existing Python -m http.server command remains valid.
• A full local docker build was not possible in this environment because the Docker daemon socket is unavailable.

Reference: the Dependabot update error on chainguard/python from April 13, 2026.