Bind-mounted filesystem as IPC between docker and host

When a containerized service and a host-side daemon both bind-mount the same data directory, the filesystem itself is the cheapest IPC channel — no HTTP server inside the container, no extra_hosts host-gateway dance, no shared secret on a new endpoint. The pattern: producer writes to a tempfile path then renames to a final path (atomic across POSIX), consumer reads the directory on its own cadence, deletes processed files, leaves failed ones for retry. Latency is bounded by the consumer poll interval, which is usually fine for non-realtime work like a new contact triggering a pull of their history. Beats the alternatives — host.docker.internal mounts, mTLS endpoints, shared queues — when the bind mount already exists for unrelated reasons. The consumer side gets retry-on-restart and a debuggable on-disk inbox for free.