security: don't install ca-certificates in the driver image

[chris-altamimi]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

The driver image installs ca-certificates, which pulls openssl and libssl3 back into the final image — undoing something the base image goes out of its way to avoid.

registry.k8s.io/build-image/debian-base deliberately ships /etc/ssl/certs/ca-certificates.crt copied in from a separate stage without the ca-certificates package, precisely so that openssl is not a dependency (see images/build/debian-base/*/Dockerfile.build in kubernetes/release). clean-install ca-certificates mount in docker/Dockerfile reverses that, because ca-certificates hard-Depends on openssl (>= 1.1.1).

Neither package is reachable from the driver. It is built with CGO_ENABLED=0, so it reads the trust bundle as a file through crypto/x509 — it never links libssl3 and never execs the openssl CLI.

mount is already present in debian-base, so that half of the install is a no-op; I've kept it so the runtime dependency stays explicit.

Why it's worth doing: openssl and libssl3 are a recurring source of CVEs in image scans, and here they are entirely unused. In a recent scan of a cluster running v1.5.0, these two packages accounted for 11 findings on the driver image — 6 of which had no fixed Debian package available at any version, so they could not be resolved by patching, only by not shipping the packages. This change removes that surface permanently rather than waiting on Debian.

Note that this repo's scan_vulns workflow currently runs govulncheck only, so OS-package findings like these don't surface in CI.

Verification: built the image before and after the change.

• openssl and libssl3 are no longer installed (82 → 79 packages)
• /etc/ssl/certs/ca-certificates.crt is still present (142 certs)
• a statically linked Go TLS probe run inside the image still completes a verified TLS 1.3 handshake to a public endpoint (2 verified chains), identically to the unmodified image
• /usr/bin/mount is still present and the driver binary still starts

Special notes for your reviewer:

There is a related, larger issue I'm raising separately against kubernetes/release: perl-base is inherited as Essential: yes from debian:bookworm-slim and is absent from debian-base's existing purge list, which affects every image built on that base. This PR is scoped only to what the driver image itself controls.

TODOs:

• squashed commits
• includes documentation
• adds unit tests

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