fix(interface): refuse container-runtime dirs and docker.sock mounts

check_mountable_dir already blocks system trees and .docker credential
dirs, but still admitted /var/run and directories that hold docker.sock.
Those bind-mount writable into the sandbox and expose the host
container runtime API. Expand the denylist and refuse runtime socket
children. Fixes #1058.

strix/interface/utils.py

@@ -1328,6 +1328,12 @@ _FORBIDDEN_MOUNT_TREES = frozenset(
         "/lib",
         "/lib64",
         "/nix/store",
+        "/run",
+        "/var/run",
+        "/private/var/run",
+        "/var/lib/docker",
+        "/var/lib/containers",
+        "/var/lib/containerd",
         "/run/current-system/sw",
         "/Applications",
         "/Library",
@@ -1339,6 +1345,16 @@ _FORBIDDEN_MOUNT_TREES = frozenset(
     }
 )
 
+# Direct children that indicate a container-runtime API socket directory.
+_FORBIDDEN_RUNTIME_SOCKET_NAMES = frozenset(
+    {
+        "docker.sock",
+        "podman.sock",
+        "containerd.sock",
+        "crio.sock",
+    }
+)
+
 # Refused themselves, but they hold projects too, so their contents are fine.
 _FORBIDDEN_MOUNT_ROOTS = frozenset(
     {
@@ -1418,6 +1434,23 @@ def check_mountable_dir(path: Path) -> None:
             "holds credentials, not code."
         )
 
+    try:
+        socket_child = next(
+            (
+                entry.name
+                for entry in resolved.iterdir()
+                if entry.name.casefold() in _FORBIDDEN_RUNTIME_SOCKET_NAMES
+            ),
+            None,
+        )
+    except OSError:
+        socket_child = None
+    if socket_child is not None:
+        raise ValueError(
+            f"Refusing to mount '{resolved}' into the sandbox: '{socket_child}' "
+            "exposes a container runtime API, not a codebase."
+        )
+
 
 def dedupe_local_targets(targets_info: list[dict[str, Any]]) -> list[dict[str, Any]]:
     result: list[dict[str, Any]] = []

tests/test_local_sources.py

@@ -166,6 +166,21 @@ def test_check_mountable_dir_rejects_system_subdirs() -> None:
         check_mountable_dir(system_subdir)
 
 
+def test_check_mountable_dir_rejects_var_run() -> None:
+    var_run = next((p for p in (Path("/var/run"), Path("/private/var/run")) if p.is_dir()), None)
+    if var_run is None:
+        pytest.skip("no /var/run on this platform")
+    with pytest.raises(ValueError, match="Refusing to mount"):
+        check_mountable_dir(var_run)
+
+
+def test_check_mountable_dir_rejects_runtime_socket_dir(tmp_path: Path) -> None:
+    sock = tmp_path / "docker.sock"
+    sock.write_text("")
+    with pytest.raises(ValueError, match="container runtime API"):
+        check_mountable_dir(tmp_path)
+
+
 def test_check_mountable_dir_accepts_a_project_under_the_home_root(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ) -> None: