fix(docker): make kubectl-delivery base image and k8s version configurable

[vicoooo26]

What this PR does / why we need it:

Makes the kubectl-delivery Dockerfile more flexible by extracting hardcoded Alpine and Kubernetes versions into build args:

• ALPINE_VERSION (default 3.22) — replaces the hardcoded alpine:3.17 in both build and final stages
• K8S_VERSION (default v1.30.7) — replaces the hardcoded ENV K8S_VERSION with an ARG, allowing override via --build-arg

This also upgrades the Alpine base from 3.17 to 3.22, bringing security fixes and updated packages. Both values can now be overridden at build time without editing the Dockerfile.

[github-actions]

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[cheyang]

Verified this locally: built the image with defaults (→ alpine 3.22.5, kubectl v1.30.7) and with --build-arg ALPINE_VERSION=3.20 --build-arg K8S_VERSION=v1.29.10 (→ alpine 3.20.10, kubectl v1.29.10). The global ARG ALPINE_VERSION declared before the first FROM is correctly referenced by both stages. Looks good.

Optional, non-blocking: the kubectl download still isn’t checksum-verified (pre-existing). Since you’re already touching this file, adding a sha256 check would be a nice hardening.

[Copilot]

Pull request overview

This PR makes the kubectl-delivery container image easier to customize by parameterizing previously hardcoded versions in the Dockerfile, while also upgrading the default Alpine base image.

Changes:

• Introduces ARG ALPINE_VERSION=3.22 and uses it for both build and runtime stages (FROM alpine:${ALPINE_VERSION}).
• Switches Kubernetes version from a hardcoded ENV to ARG K8S_VERSION=v1.30.7 so it can be overridden at build time.

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