[Agent] Replace dockerapi container with Redis-based control bus

data/Dockerfiles/agent/README.md

@@ -0,0 +1,16 @@
+# mailcow-agent
+
+Each mailcow service container (postfix, dovecot, …) runs `mailcow-agent` as
+ENTRYPOINT. It supervises the original service main process and exposes its
+control commands over a Redis Pub/Sub bus:
+
+- `mailcow.control.<service>` — request channel (Backend → Agent)
+- `mailcow.reply.<request_id>` — per-request reply channel
+- `mailcow.events.<topic>` — broadcast events
+- `mailcow.nodes.<service>` (ZSET) + `mailcow.node.<service>.<node_id>` (HASH) — heartbeat registry
+- `mailcow.stats.<service>.<node_id>` (HASH) — per-node cpu/memory stats
+
+Service behaviour is selected via `MAILCOW_AGENT_SERVICE=<service>`. The main
+process command is configured via `MAILCOW_AGENT_MAIN_CMD` (string, executed via
+`sh -c` so existing entrypoints/supervisord commands keep working).
+