feat(images): build on rootless BuildKit over mTLS; verification moves into Containerfile verify stages

Operator decision on PR #81: no CI job may reach a privileged daemon. The act runner is untouched.

- platform/buildkit (+ clusters/base/buildkit.yaml): moby/buildkit v0.34.1-rootless pinned by
  digest, uid 1000, not privileged; caps drop ALL + SETUID/SETGID, seccomp/AppArmor Unconfined,
  read-only rootfs (each measured under podman). cert-manager CA + server/client certs, ClusterIP
  :1234, default-deny with ingress only from the act-runner pod (DinD masquerades jobs to it).
  No Kyverno exception: every applied policy matches app-namespace=true only.
- Each Containerfile names its shipped stage and ends in a `verify` stage holding every former
  `docker run` assertion (uid 65532, RUN --network=none, writable-path inventory standing in for
  the read-only rootfs). The old `&& echo` checks that could never fail are real assertions now.
- images.sh drives buildkitd with a pinned buildx (sha256-checked) via bake; PRs build verify
  targets with agent-runner FROM the in-run toolbox (target: context); main verifies, then pushes.
  Fails loudly when the builder secrets are absent or buildkitd is unreachable.
- infra/gitea/buildkit-client-secret.sh copies the client cert into Actions secrets.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

conformance/incluster/README.md

@@ -61,10 +61,12 @@ credentials — the Job's own scoped ServiceAccount does (R-SEC-02).
   > digest-pinned to the conformance-runner image, `command` exactly
   > `/usr/local/bin/conformance-clone`, and **no `args` at all**. `ephemeralContainers` and
   > `env`/`envFrom` were the same shape of hole and are closed in the same policy.
-- `build-image.sh` — builds the above and **verifies the toolchain by running it** inside the
-  image as uid 65532 with `--read-only`. It **never pushes**, and neither does anyone from a
-  workstation: CI (`.gitea/workflows/images.yaml`) runs this same script, and on a push to
-  `main` publishes the verified image under `sha-<commit>` and `main`. Renovate then moves
+- `build-image.sh` — builds the above and **verifies the toolchain by running it**: the
+  Containerfile's `verify` stage executes every tool and every `conformance-clone` refusal as
+  uid 65532 with no network, at build time. It **never pushes**, and neither does anyone from a
+  workstation: CI (`.gitea/workflows/images.yaml`) builds the same Containerfile and `verify`
+  stage on rootless BuildKit (`platform/buildkit/`), and on a push to `main` publishes the
+  verified image under `sha-<commit>` and `main`. Renovate then moves
   `CONFORMANCE_RUNNER_IMAGE_DIGEST` in `conformance/versions.env` onto the registry-returned
   digest in a pull request (that file is a sensitive path, so the bump needs an
   administrator's `/approve-override`).