Suggest disabling pip cache in containers

When building containers (like docker or podman) the layer system already handles the caching.
Not disabling pip's cache could result in the duplication of the size of the images.

I know this sujbject is not specially relevant to the documentation, so the comment is fairly small.
Duplication of pip's cache is a recurrent problem in many Python images

Author: federicober

docs/html/topics/caching.md

@@ -140,6 +140,6 @@ The {ref}`pip cache` command can be used to manage pip's cache.
 
 pip's caching behaviour is disabled by passing the `--no-cache-dir` option.
 
-It is, however, recommended to **NOT** disable pip's caching. Doing so can
+It is, however, recommended to **NOT** disable pip's caching (except for building containerized appplications). Doing so can
 significantly slow down pip (due to repeated operations and package builds)
 and result in significantly more network usage.