Update rule S6587: add multi-stage build exceptions (#4539)

rules/S6587/docker/rule.adoc

@@ -8,6 +8,10 @@ The package index is redundant for the correct operation of the installed softwa
 Storing an index also increases the size of the Docker image.
 It should be reduced to speed up deployments and reduce storage and bandwidth.
 
+=== Exceptions
+
+In multi-stage builds, the rule only checks instructions that are part of the final image.
+
 == How to fix it
 
 === Code examples