Update rule S7031: add multi-stage build exceptions

rules/S7031/docker/rule.adoc

@@ -8,6 +8,10 @@ This practice can make Docker images more efficient and easier to manage. +
 Each layer in a Docker image is essentially a change to the image, like a version control system. +
 So, fewer layers mean fewer changes to track, which simplifies management and troubleshooting.
 
+=== Exceptions
+
+In multi-stage builds, the rule only scans instructions that are part of the final image.
+
 == How to fix it
 
 === Code examples