docs: document sandbox media staging

docs/docker.md

@@ -73,6 +73,7 @@ container. The gateway stays on your host, but the tool execution is isolated:
 - one container per session (hard wall)
 - per-session workspace folder mounted at `/workspace`
 - allow/deny tool policy (deny wins)
+- inbound media is copied into the sandbox workspace (`media/inbound/*`) so tools can read it
 
 ### Default behavior