docs: remove docs for —-from=index

Naming stages is the preferred method for using
multi-stage builds.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
(cherry picked from commit 5dd9bd4c2cead98c1b183402a172dd0d488703ae)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: 9152bf265ee53a60a6d141cc294bf14cd899c7f9
Component: cli

Author: tonistiigi

components/cli/docs/reference/builder.md

@@ -666,7 +666,7 @@ the [*Public Repositories*](https://docs.docker.com/engine/tutorials/dockerrepos
   instructions.
 - Optionally a name can be given to a new build stage by adding `AS name` to the
   `FROM` instruction. The name can be used in subsequent `FROM` and
-  `COPY --from=<name|index>` instructions to refer to the image built in this stage.
+  `COPY --from=<name>` instructions to refer to the image built in this stage.
 - The `tag` or `digest` values are optional. If you omit either of them, the
   builder assumes a `latest` tag by default. The builder returns an error if it
   cannot find the `tag` value.
@@ -1311,12 +1311,11 @@ no lookup and does not depend on container root filesystem content.
 > If you build using STDIN (`docker build - < somefile`), there is no
 > build context, so `COPY` can't be used.
 
-Optionally `COPY` accepts a flag `--from=<name|index>` that can be used to set
+Optionally `COPY` accepts a flag `--from=<name>` that can be used to set
 the source location to a previous build stage (created with `FROM .. AS <name>`)
-that will be used instead of a build context sent by the user. The flag also
-accepts a numeric index assigned for all previous build stages started with
-`FROM` instruction. In case a build stage with a specified name can't be found an
-image with the same name is attempted to be used instead.
+that will be used instead of a build context sent by the user. In case a build
+stage with a specified name can't be found an image with the same name is
+attempted to be used instead.
 
 `COPY` obeys the following rules: