doc: walkthrough on Sandbox NodeLocal DNS with NetworkPolicy

[vicentefb]

What this PR does / why we need it:

This document addition provides concrete, reproducible GKE cluster manifests and commands to demonstrate how GKE NodeLocal DNSCache (169.254.20.10) operates under GKE Sandbox (gVisor) security isolation:

1. The gVisor eBPF Socket Bypass gotcha: On standard GKE pods (without gVisor), DNS queries targeting 169.254.20.10 are intercepted and redirected in-memory by host-level eBPF socket hooks (cgroup/connect), completely bypassing the virtual network interface (eth0) and any egress NetworkPolicy rules.
2. Egress Policy Dependency: Because gVisor isolates all socket operations inside its virtualized user-space netstack (gonet), host-level socket hooks are bypassed. The query is forced out as a standard L3 packet over eth0, making it strictly subject to your egress rules.
3. Reproducible Scenarios: Includes step-by-step configurations illustrating how strict default egress blocks (which include link-local 169.254.0.0/16 ranges) drop DNS cache traffic, and how explicitly unblocking the link-local range natively enables instant ~10ms DNS resolution without hostAliases or custom external servers.

Which issue(s) this PR is related to:

Release Note

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[Copilot]

Pull request overview

Adds a new example walkthrough documenting how GKE NodeLocal DNSCache (169.254.20.10) behaves for gVisor (runtimeClassName: gvisor) sandboxes under Agent Sandbox “Secure by Default” NetworkPolicy management, including reproducible manifests and verification commands.

Changes:

• Introduces a step-by-step PoC with three scenarios (default blocked, kube-dns VIP blocked, link-local unblocked success).
• Documents the underlying reason (gVisor netstack bypassing host-level eBPF socket redirection) and why egress rules matter for link-local DNS.

[aditya-shantanu]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aditya-shantanu, vicentefb

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