scripts/container_scan.sh: add container image CVE scanning

[kairosci]

Add a script, Makefile target, and CI workflow to scan the container base image with Trivy for OS-level CVEs. etcd is built with CGO_ENABLED=0 and the existing govulncheck+CodeQL pipeline only covers Go vulnerabilities, so OS-level CVEs in the distroless base image were not detected. Closes #21252

[k8s-ci-robot]

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[k8s-ci-robot]

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[kairosci]

cc @ahrtr @ivanvc

[ahrtr]

@ArkaSaha30 @ivanvc I believe we already have such CVE scan against container image, correct?