1. Fix dead URL to [Dockerfile best practices](https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#leverage-build-cache). 2. Add missing information about cache invalidation by `COPY`. It works in the same way as in the case of `ADD`. Informing only about the `ADD`s behavior is misleading as one can infer that these two directives differ in this regard. 3. Add missing info on RUN cache invalidation by COPY Signed-off-by: Maciej Kalisz <maciej.d.kalisz@gmail.com>