SONARIAC-1701 Update RSPEC description for S6587

rules/S6587/docker/rule.adoc

@@ -131,6 +131,21 @@ 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 as the installation in the same `RUN` instruction.
+
+The following code incorrectly cleans the cache:
+[source,docker]
+----
+RUN apt-get install -y git
+RUN apt-get clean && rm -rf /var/lib/apt/lists/*
+----
+It should be written as:
+[source,docker]
+----
+RUN apt-get install -y git \
+  && apt-get clean && rm -rf /var/lib/apt/lists/*
+----
+
 == Resources
 === Documentation