SONARIAC-1701 Update RSPEC description for S6587 (#4562)

* SONARIAC-1701 Update RSPEC description for S6587

* Address review comments

rules/S6587/docker/rule.adoc

@@ -131,6 +131,20 @@ They are not removed by the `clean` command, so they need to be removed manually
 Alternatively, store the cache in a dedicated cache mount. A cache mount can be created by adding a flag `--mount type=cache` to the `RUN` command. +
 This will store the cache in a Docker volume, which will be persisted between builds making the build faster.
 
+Also, each `RUN` instruction creates a new layer, and any changes made in one layer are not visible in the next. Thus, the cache should be removed in the same layer (i.e., the same `RUN` instruction) as the installation.
+
+The following code incorrectly cleans the cache:
+[source,docker]
+----
+RUN apt-get install nginx
+RUN apt-get clean
+----
+It should be written as:
+[source,docker]
+----
+RUN apt-get install nginx && apt-get clean
+----
+
 == Resources
 === Documentation