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]>

platform/buildkit/networkpolicy.yaml

@@ -0,0 +1,88 @@
+---
+# Default-deny for the namespace, then two narrow allows.
+#
+# WHO MAY CONNECT — AND WHY THE ACT-RUNNER POD IS THE RIGHT SOURCE (measured 2026-10-29).
+# CI jobs do not run as pods. act_runner starts each job as a container on its DinD sidecar, on a
+# docker bridge network INSIDE the act-runner pod's network namespace, and dockerd masquerades it:
+#   kubectl -n gitea-actions exec deploy/act-runner -c dind -- iptables -t nat -S POSTROUTING
+#     -A POSTROUTING -s 172.18.0.0/16 ! -o br-c0f13d041ad6 -j MASQUERADE
+#     -A POSTROUTING -s 172.17.0.0/16 ! -o docker0 -j MASQUERADE
+# So a job's connection leaves with the act-runner POD's IP and therefore Cilium's identity for
+# that pod (namespace gitea-actions, app.kubernetes.io/name=act-runner). That is exactly what the
+# ingress rule names. It cannot distinguish one job from another, or a job from act_runner
+# itself — per-job authentication is the mTLS client certificate, not this policy. What the
+# policy adds is that NOTHING ELSE in the cluster can even open the port.
+apiVersion: networking.k8s.io/v1
+kind: NetworkPolicy
+metadata:
+  name: default-deny
+  namespace: buildkit
+  labels:
+    homelab.keithjackson.dev/baseline: default-deny
+spec:
+  podSelector: {}
+  policyTypes:
+    - Ingress
+    - Egress
+  ingress: []
+  egress:
+    - to:
+        - namespaceSelector:
+            matchLabels:
+              kubernetes.io/metadata.name: kube-system
+      ports:
+        - protocol: UDP
+          port: 53
+        - protocol: TCP
+          port: 53
+---
+apiVersion: cilium.io/v2
+kind: CiliumNetworkPolicy
+metadata:
+  name: buildkitd-allow-ci-ingress
+  namespace: buildkit
+  labels:
+    app.kubernetes.io/name: buildkitd
+spec:
+  endpointSelector:
+    matchLabels:
+      app.kubernetes.io/name: buildkitd
+  ingress:
+    - fromEndpoints:
+        - matchLabels:
+            io.kubernetes.pod.namespace: gitea-actions
+            app.kubernetes.io/name: act-runner
+      toPorts:
+        - ports:
+            - { port: "1234", protocol: TCP }
+---
+# WHAT A BUILD NEEDS TO REACH. Build steps download (dnf mirrors, GitHub releases, dl.k8s.io,
+# npm, PyPI, Fedora/Docker registries) and the result is pushed to the Gitea registry.
+#   * world 443/80: the downloads. 80 because Fedora's mirror list includes plain-http mirrors
+#     (dnf verifies package GPG signatures, not transport).
+#   * ingress (the Gateway's Envoy, reserved:ingress): the push. gitea.home.keithjackson.dev
+#     resolves in-cluster to the Gateway VIP (split-horizon CoreDNS), and Cilium hands that
+#     traffic to the Gateway, whose identity is `ingress` — the same identity gitea's own
+#     ingress allow admits.
+# Nothing else in the cluster: not the API server, not other pods. A build step is code from a
+# pull request.
+apiVersion: cilium.io/v2
+kind: CiliumNetworkPolicy
+metadata:
+  name: buildkitd-allow-egress
+  namespace: buildkit
+  labels:
+    app.kubernetes.io/name: buildkitd
+spec:
+  endpointSelector:
+    matchLabels:
+      app.kubernetes.io/name: buildkitd
+  egress:
+    - toEntities:
+        - world
+      toPorts:
+        - ports:
+            - { port: "443", protocol: TCP }
+            - { port: "80", protocol: TCP }
+    - toEntities:
+        - ingress