Document dind and cli variants

Author: yosifkit

docker/variant-cli.md

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+## `%%IMAGE%%:<version>-cli`
+
+This image contains the Docker client command line interface (CLI) and Docker CLI plugins like `buildx` and `compose`. This is useful if you need to interact with a remote Docker engine but aren't planning to run the Docker engine in the container.

docker/variant-dind.md

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+## `%%IMAGE%%:<version>`, `%%IMAGE%%:<version>-dind`
+
+The default variant is the Docker in Docker variant. It contains the Docker engine as well as the Docker CLI and plugins that are included in the `cli` variant. It is useful for running Docker in Docker and for interacting with a Docker engine via the Docker CLI.

docker/variant.md

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+# Image Variants
+
+The `%%IMAGE%%` images come in many flavors, each designed for a specific use case.
+
+**Note:**: The `*stable`, and `*test` tags have been deprecated since June 2020 (see [docker-library/docker#179](https://github.com/docker-library/docker/pull/179)) and were no longer updated in December 2020 when Docker 20.10 was released. The alternative to `stable` is to use `latest` (there was no difference when it was deprecated). You can also use the `cli`, and `dind` tags to get the latest Docker CLI or engine, respectively. The alternative to the `test` tags is to use the `rc` tag or a tag with `-rc` in it. The `rc` tag is only updated when there is an active pre-release and will not point to the same thing as `latest`.