Revert "Since `COPY` has been deprecated in the recent release of Docker I updated the best practices section to inform users that `ADD` is the way forward and to not use `COPY`."

COPY was not deprecated.

This reverts commit 3148063af1.

Signed-off-by: Tibor Vass <tibor@docker.com>

docs/sources/articles/dockerfile_best-practices.md

@@ -248,7 +248,12 @@ auto-magically bump the version of the software in your container.
 
 ### [`ADD`](https://docs.docker.com/reference/builder/#add) or [`COPY`](https://docs.docker.com/reference/builder/#copy)
 
-Note that `COPY` has been deprecated in the most recent release of Docker and you'll be prompted as such when you use it. It is recommended to use `ADD` from this point onward.
+Although `ADD` and `COPY` are functionally similar, generally speaking, `COPY`
+is preferred. That’s because it’s more transparent than `ADD`. `COPY` only
+supports the basic copying of local files into the container, while `ADD` has
+some features (like local-only tar extraction and remote URL support) that are
+not immediately obvious. Consequently, the best use for `ADD` is local tar file
+auto-extraction into the image, as in `ADD rootfs.tar.xz /`.
 
 If you have multiple `Dockerfile` steps that use different files from your
 context, `COPY` them individually, rather than all at once. This will ensure that