feat(macos): run containers inside a bottle via guest-local podman

Adds the `nested_containers` bottle flag. On the macOS backend it starts
a rootless podman service inside the bottle and exposes its
Docker-compatible API socket, so the agent still runs `docker` and
`docker compose`. No host daemon socket is mounted and the guest gains
no capabilities; backends that cannot run a guest-local engine reject
the flag in the shared prepare template rather than ignoring it.

Podman rather than rootless Docker because Apple Container's capability
bounding set omits CAP_SYS_ADMIN, which the kernel requires to write a
multi-range uid_map via newuidmap. With no subordinate UID range podman
falls back to a single-UID self-mapping an unprivileged process may
write itself, so the image build strips /etc/subuid and /etc/subgid
entries rather than adding them.

That mapping is also why nested containers are not an isolation layer:
root inside one is the agent user outside it. They are a build/test
convenience; the bottle remains the boundary.

Ports the spike branch onto main, renaming docker_access — it implied
Docker and granted access to nothing on the host — and drops podman
from the derived layer now that every built-in image ships it (#451).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/unit/test_backend_prepare.py

@@ -21,7 +21,7 @@ from bot_bottle.backend.firecracker import FirecrackerBottleBackend
 from bot_bottle.manifest import ManifestIndex
 
 
-def _manifest() -> ManifestIndex:
+def _manifest(*, nested_containers: bool = False) -> ManifestIndex:
     return ManifestIndex.from_json_obj({
         "bottles": {
             "dev": {
@@ -29,6 +29,7 @@ def _manifest() -> ManifestIndex:
                     "LITERAL_ENV": "literal-value",
                     "FORWARDED_ENV": "${HOST_SECRET_ENV}",
                 },
+                "nested_containers": nested_containers,
             },
         },
         "agents": {
@@ -41,9 +42,11 @@ def _manifest() -> ManifestIndex:
     })
 
 
-def _spec(tmp: Path, *, identity: str) -> BottleSpec:
+def _spec(
+    tmp: Path, *, identity: str, nested_containers: bool = False,
+) -> BottleSpec:
     return BottleSpec(
-        manifest=_manifest(),
+        manifest=_manifest(nested_containers=nested_containers),
         agent_name="demo",
         copy_cwd=False,
         user_cwd=str(tmp),
@@ -113,6 +116,42 @@ class TestFirecrackerPrepare(_FakeStateMixin, unittest.TestCase):
         self.assertEqual({"FORWARDED_ENV": "secret-value"}, plan.forwarded_env)
 
 
+class TestNestedContainersRejection(_FakeStateMixin, unittest.TestCase):
+    """A backend with no guest-local engine must refuse the flag outright.
+
+    Ignoring it would leave the agent without `docker`, and the only ways to
+    fake it here (a host daemon socket, a privileged container) are what
+    issue #392 rules out.
+    """
+
+    def test_docker_backend_refuses_the_flag(self) -> None:
+        backend = DockerBottleBackend()
+        spec = _spec(
+            Path(self.tmp.name), identity="demo-docker", nested_containers=True,
+        )
+        with (
+            patch(
+                "bot_bottle.backend.resolve_common.die", side_effect=RuntimeError,
+            ) as die,
+            self.assertRaises(RuntimeError),
+        ):
+            backend.prepare(spec, Path(self.tmp.name) / "stage")
+        self.assertIn("nested_containers", die.call_args.args[0])
+        self.assertIn("docker", die.call_args.args[0])
+
+    def test_firecracker_backend_refuses_the_flag(self) -> None:
+        backend = FirecrackerBottleBackend()
+        spec = _spec(Path(self.tmp.name), identity="demo-fc", nested_containers=True)
+        with (
+            patch(
+                "bot_bottle.backend.resolve_common.die", side_effect=RuntimeError,
+            ) as die,
+            self.assertRaises(RuntimeError),
+        ):
+            backend.prepare(spec, Path(self.tmp.name) / "stage")
+        self.assertIn("firecracker", die.call_args.args[0])
+
+
 class TestMintSlug(unittest.TestCase):
     def _spec(self, *, label: str = "", identity: str = "") -> BottleSpec:
         manifest = _manifest()