add container info

README.md

@@ -62,7 +62,9 @@ For the last one the scanner asks the Docker Engine API on the Unix socket for
 `/containers/json` and scans each `Image` reference, de-duplicated and in
 listing order. Stopped containers are not included. The scanner's own container
 image is skipped so a scheduled run does not report on itself; set
-`INCLUDE_SELF=true` to include it.
+`INCLUDE_SELF=true` to include it. Scan log lines name the running container(s)
+that use each image, for example
+`Scanning nginx:1.27 (used by container web)`.
 
 The socket is looked up in this order: