Upgrade the build image from ubuntu to debian for ISO buildroot

[afbjorklund]

ubuntu 20.04:
https://github.com/kubernetes/minikube/blob/master/deploy/iso/minikube-iso/Dockerfile

debian 12:
https://github.com/buildroot/buildroot/blob/2025.02.x/support/docker/Dockerfile

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This should also make it multi-arch, and the documentation can updated accordingly.

Machine with x86_64 CPU
Ubuntu 22.04.5 LTS (Jammy Jellyfish)

• docs: Fix instructions for building iso #20830