Fix release-builder Git dependency and isolate guardrail fixture sources

Assistant: codex
Assistant-Model: gpt-6-astra
Assistant-Session: 01a15374-f59f-7a31-86d2-d23b5d2ed181

Containerfile

@@ -24,6 +24,8 @@ FROM runtime-base AS local-artifact
 COPY --chown=101:101 build/ /usr/share/nginx/html/
 
 FROM docker.io/library/python@sha256:d09d15e60962ca365d1cd544a48773bac9d33f2fb1b00f2aa0deec78ade7dc31 AS release-builder
+# Staged-source review tests exercise real Git index semantics. Builder only.
+RUN apk add --no-cache git
 ARG SOURCE_SET_DIGEST
 ENV POLICY_NEXUS_SOURCE_ROOT=/workspace/_sources
 WORKDIR /workspace/policy-nexus

tests/test_guardrails.py

@@ -2,10 +2,12 @@ from __future__ import annotations
 
 import json
 import hashlib
+import os
 from pathlib import Path
 import sys
 import tempfile
 import unittest
+from unittest import mock
 
 ROOT = Path(__file__).parents[1]
 sys.path.insert(0, str(ROOT / 'tools'))
@@ -20,6 +22,10 @@ CANON_FIXTURE = '---\nid: canon-build-mode-change-authorization\ntitle: Build-Mo
 
 class GuardrailTest(unittest.TestCase):
     def setUp(self):
+        # Synthetic fixtures must not inherit the CI release source lock.
+        source_env = mock.patch.dict(os.environ, {"POLICY_NEXUS_SOURCE_ROOT": ""})
+        source_env.start()
+        self.addCleanup(source_env.stop)
         self.source = CANON_FIXTURE
         self.rule = extract_rule(self.source)
         self.record = {'categories': [], 'diff_ref': 'reviewed.patch', 'rollback': 'revert source/image',