docs: explain when RUN cache gets invalidated

This adds a few lines to the RUN Dockerfile docs to explain how to
bypass the RUN caching and how ADD can also invalidate the cache for
these instructions.

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #5816 from unclejack/docs_run_cache

docs: explain when RUN cache gets invalidated