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>

bot_bottle/backend/__init__.py

@@ -302,6 +302,11 @@ class BottleBackend(ABC, Generic[PlanT, CleanupT]):
 
     name: str
 
+    # Whether this backend can run a container engine *inside* the bottle.
+    # Backends that cannot must reject `nested_containers: true` rather than
+    # reach for a host daemon socket (issue #392).
+    supports_nested_containers: bool = False
+
     def prepare(self, spec: BottleSpec, stage_dir: Path) -> PlanT:
         """Template method: run cross-backend host-side validation, then
         delegate to the subclass's `_resolve_plan` for the
@@ -315,12 +320,16 @@ class BottleBackend(ABC, Generic[PlanT, CleanupT]):
             prepare_egress,
             prepare_git_gate,
             prepare_supervise,
+            reject_nested_containers,
             resolve_manifest_dockerfile,
             write_launch_metadata,
         )
 
         manifest = self._validate(spec)
 
+        if not self.supports_nested_containers:
+            reject_nested_containers(self.name, manifest)
+
         self._preflight()
 
         from ..git_gate_host_key import preflight_host_keys