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.

tests/test_platforms.py

@@ -0,0 +1,82 @@
+"""Which platform images a published reference has, for bin/sign.sh.
+
+base-os publishes an index of amd64 and arm64; netork's dev images are a single
+amd64 manifest, and a release joins arm64 to it under a new index. Every
+platform image gets its own CIS result and SBOM, so sign.sh needs each one's
+architecture and digest, whatever shape the reference has.
+"""
+
+from __future__ import annotations
+
+import json
+
+import pytest
+
+from image_tools.platforms import platforms
+
+AMD = "sha256:" + "a" * 64
+ARM = "sha256:" + "b" * 64
+ATT = "sha256:" + "c" * 64
+SELF = "sha256:" + "d" * 64
+
+
+def _index(*manifests: dict) -> str:
+    return json.dumps(
+        {"mediaType": "application/vnd.oci.image.index.v1+json", "manifests": list(manifests)}
+    )
+
+
+def _entry(digest: str, arch: str | None, os: str = "linux") -> dict:
+    entry: dict = {"digest": digest, "mediaType": "application/vnd.oci.image.manifest.v1+json"}
+    if arch:
+        entry["platform"] = {"os": os, "architecture": arch}
+    return entry
+
+
+def test_an_index_names_each_linux_platform():
+    raw = _index(_entry(AMD, "amd64"), _entry(ARM, "arm64"))
+
+    assert platforms(raw, "{}", SELF) == [("amd64", AMD), ("arm64", ARM)]
+
+
+def test_buildx_attestation_manifests_are_no_platform():
+    raw = _index(_entry(AMD, "amd64"), _entry(ATT, "unknown", os="unknown"), _entry(ATT, None))
+
+    assert platforms(raw, "{}", SELF) == [("amd64", AMD)]
+
+
+def test_a_single_manifest_is_its_own_platform():
+    raw = json.dumps({"mediaType": "application/vnd.oci.image.manifest.v1+json", "layers": []})
+    image = json.dumps({"architecture": "amd64", "os": "linux"})
+
+    assert platforms(raw, image, SELF) == [("amd64", SELF)]
+
+
+def test_a_docker_v2_manifest_list_counts_as_an_index():
+    raw = json.dumps(
+        {
+            "mediaType": "application/vnd.docker.distribution.manifest.list.v2+json",
+            "manifests": [_entry(ARM, "arm64")],
+        }
+    )
+
+    assert platforms(raw, "{}", SELF) == [("arm64", ARM)]
+
+
+@pytest.mark.parametrize(
+    "raw",
+    [
+        _index(_entry(ATT, "unknown", os="unknown")),
+        json.dumps({"mediaType": "application/vnd.oci.image.manifest.v1+json"}),
+    ],
+    ids=["index without an image", "manifest without an architecture"],
+)
+def test_nothing_to_sign_is_an_error(raw):
+    with pytest.raises(ValueError):
+        platforms(raw, "{}", SELF)
+
+
+def test_an_architecture_not_built_here_is_an_error():
+    """Only amd64 and arm64 are built (#934); anything else would be signed blind."""
+    with pytest.raises(ValueError, match="riscv64"):
+        platforms(_index(_entry(AMD, "riscv64")), "{}", SELF)