Harden hosted Docker install tree against self-modification (#47490)

* Harden hosted Docker install tree

* Document hosted Docker immutable install tree

tests/tools/test_dockerfile_immutable_install.py

@@ -0,0 +1,61 @@
+"""Contract tests for the Docker image's immutable /opt/hermes install tree."""
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+REPO_ROOT = Path(__file__).resolve().parents[2]
+DOCKERFILE = REPO_ROOT / "Dockerfile"
+
+
+def _dockerfile_text() -> str:
+    return DOCKERFILE.read_text()
+
+
+def test_dockerfile_makes_opt_hermes_root_owned_and_non_writable() -> None:
+    text = _dockerfile_text()
+
+    assert "COPY --chown=hermes:hermes . ." not in text
+    assert "COPY . ." in text
+    assert "chown -R root:root /opt/hermes" in text
+    assert "chmod -R a+rX /opt/hermes" in text
+    assert "chmod -R a-w /opt/hermes" in text
+
+    immutable_block = re.search(
+        r"RUN mkdir -p /opt/hermes/bin && \\\n"
+        r"(?:.*\\\n)+?"
+        r"\s+chmod -R a-w /opt/hermes",
+        text,
+    )
+    assert immutable_block, "Dockerfile must lock /opt/hermes after installing code/deps"
+
+
+def test_dockerfile_keeps_mutable_state_under_opt_data() -> None:
+    text = _dockerfile_text()
+
+    assert "ENV HERMES_HOME=/opt/data" in text
+    assert "ENV HERMES_WRITE_SAFE_ROOT=/opt/data" in text
+    assert 'VOLUME [ "/opt/data" ]' in text
+
+
+def test_dockerfile_disables_runtime_install_mutations() -> None:
+    text = _dockerfile_text()
+
+    assert "ENV PYTHONDONTWRITEBYTECODE=1" in text
+    assert "ENV HERMES_DISABLE_LAZY_INSTALLS=1" in text
+    assert "HERMES_TUI_DIR=/opt/hermes/ui-tui" in text
+
+
+def test_dockerfile_does_not_chown_install_trees_to_hermes() -> None:
+    text = _dockerfile_text()
+    forbidden_patterns = (
+        r"chown\s+-R\s+hermes:hermes\s+/opt/hermes/\.venv",
+        r"chown\s+-R\s+hermes:hermes\s+/opt/hermes/ui-tui",
+        r"chown\s+-R\s+hermes:hermes\s+/opt/hermes/gateway",
+        r"chown\s+-R\s+hermes:hermes\s+/opt/hermes/node_modules",
+    )
+    for pattern in forbidden_patterns:
+        assert not re.search(pattern, text), (
+            "runtime install trees under /opt/hermes must stay immutable; "
+            f"found forbidden pattern {pattern!r}"
+        )