feat: judge, sign and verify netOrk's images in one place (NetOrk/netork#1390)

The CIS gate, the SBOM, the cosign signature and the matcher's verdict lived
in NetOrk/base-os, the head of the chain. Every link below it -- base-python,
engine, ui, satellite, the service images -- needs the same, so the tools
move here, and every image repository pins them by commit.

- image_tools: cis.py, cis_os.sh, sbom.py and verify.py from base-os. The
  image's own repository holds its cis.toml and packages.allow; both default
  to the working directory.
- bin/sign.sh signs whatever platforms a reference holds: base-os's index of
  amd64 and arm64 (EXPECT_PLATFORMS) or a dev image's single manifest
  (image_tools/platforms.py).
- bin/verify-base.sh checks every FROM image of a Dockerfile against
  cosign.pub before a build; one without a digest is refused
  (image_tools/dockerfile.py).
- bin/install-cosign.sh installs cosign v3.1.3 for amd64 and arm64 runners,
  checked against the release checksums.
- actions/verify-base, judge, sign wrap them for the workflows.
- cosign.pub is the public half of the COSIGN_PRIVATE_KEY_B64 org secret,
  the key rotated on 2026-10-22.

bin/sign.sh

@@ -0,0 +1,60 @@
+#!/bin/bash
+# sign.sh <image>@<digest> <dir>: sign a published image and attach what it was judged by.
+#
+# The reference and every platform image behind it get a cosign signature made
+# with the key in COSIGN_PRIVATE_KEY_B64 (its password in COSIGN_PASSWORD). Each
+# platform image gets its CIS result (<dir>/cis-<arch>.json, image_tools/cis.py)
+# and its CycloneDX SBOM (<dir>/sbom-<arch>.cdx.json, image_tools/sbom.py) as
+# in-toto attestations. This takes the place of Docker Content Trust (CIS Docker
+# 4.5), as cosign does for Docker's Hardened Images.
+#
+# The platforms are whatever the reference holds (image_tools/platforms.py): an
+# index of amd64 and arm64, or one manifest. EXPECT_PLATFORMS ("amd64 arm64")
+# makes any other set an error.
+#
+# Nothing goes to a public transparency log: cosign-signing.json names none, so
+# the names in this registry stay inside. Everything is verified against
+# cosign.pub before it returns.
+set -euo pipefail
+: "${COSIGN_PRIVATE_KEY_B64:?is empty: the secret is missing}" "${COSIGN_PASSWORD:?is empty: the secret is missing}"
+# The key comes base64 in one line: the runner prints a step's env and masks a
+# secret only where it appears whole, and a PEM key's lines did not (2026-10-22).
+COSIGN_PRIVATE_KEY=$(printf '%s' "$COSIGN_PRIVATE_KEY_B64" | base64 -d)
+export COSIGN_PRIVATE_KEY
+ref=$1
+dir=$(cd "$2" && pwd)
+cd "$(dirname "$0")/.."
+
+CIS_TYPE=https://netork.io/attestations/cis/v1
+# quietly <command>: cosign's chatter only when it fails.
+quietly() { local out; out=$("$@" 2>&1) || { echo "$out" >&2; return 1; }; }
+
+case $ref in *@sha256:*) ;; *) echo "$ref is not pinned by digest" >&2; exit 1 ;; esac
+image=${ref%@*}
+case ${image##*/} in *:*) image=${image%:*} ;; esac # the tag, not a registry port
+
+quietly cosign sign --yes --recursive --key env://COSIGN_PRIVATE_KEY \
+  --signing-config cosign-signing.json "$ref"
+quietly cosign verify --key cosign.pub --insecure-ignore-tlog "$ref"
+
+raw=$(mktemp) config=$(mktemp)
+trap 'rm -f "$raw" "$config"' EXIT
+docker buildx imagetools inspect "$ref" --raw >"$raw"
+docker buildx imagetools inspect "$ref" --format '{{json .Image}}' >"$config"
+platforms=$(python3 -m image_tools.platforms "${ref#*@}" "$raw" "$config")
+if [ -n "${EXPECT_PLATFORMS:-}" ]; then
+  got=$(echo "$platforms" | cut -d' ' -f1 | sort | xargs)
+  want=$(echo "$EXPECT_PLATFORMS" | tr ' ' '\n' | sort | xargs)
+  [ "$got" = "$want" ] || { echo "expected $want in $ref, found $got" >&2; exit 1; }
+fi
+
+while read -r arch digest; do
+  for attestation in "$CIS_TYPE cis-$arch.json" "cyclonedx sbom-$arch.cdx.json"; do
+    read -r type file <<<"$attestation"
+    quietly cosign attest --yes --key env://COSIGN_PRIVATE_KEY --signing-config cosign-signing.json \
+      --type "$type" --predicate "$dir/$file" "$image@$digest"
+    quietly cosign verify-attestation --key cosign.pub --insecure-ignore-tlog \
+      --type "$type" "$image@$digest"
+  done
+  echo "signed and attested: $image@$digest ($arch)"
+done <<<"$platforms"