docs: add cache storage backend docs

This fleshes out the documentation to include information along with
examples and detailed use cases for the cache backends. The general
format and style follows from the style of the build driver docs.

Eventually, these docs will be included on docs.docker.com along with
the rest of the build docs.

Signed-off-by: Justin Chadwell <me@jedevc.com>

docs/guides/cache/inline.md

@@ -0,0 +1,45 @@
+# Inline cache storage
+
+The `inline` cache store is the simplest way to get an external cache and is
+easy to get started using if you're already building and pushing an image.
+However, it doesn't scale as well to multi-stage builds as well as the other
+drivers do and it doesn't offer separation between your output artifacts and
+your cache output. This means that if you're using a particularly complex build
+flow, or not exporting your images directly to a registry, then you may want to
+consider the [registry](./registry.md) cache.
+
+To export your cache using `inline` storage, we can pass `type=inline` to the
+`--cache-to` option:
+
+```console
+$ docker buildx build --push -t <user>/<image> --cache-to type=inline .
+```
+
+Alternatively, you can also export inline cache by setting the build-arg
+`BUILDKIT_INLINE_CACHE`, instead of using the `--cache-to` flag:
+
+```console
+$ docker buildx build --push -t <user>/<image> --arg BUILDKIT_INLINE_CACHE=1 .
+```
+
+To import the resulting cache on a future build, we can pass `type=registry` to
+`--cache-from` which lets us extract the cache from inside a docker image:
+
+```console
+$ docker buildx build --push -t <user>/<image> --cache-from type=registry,ref=<user>/<image> .
+```
+
+Most of the time, you'll want to have each build both import and export cache
+from the cache store - to do this, specify both `--cache-to` and `--cache-from`:
+
+```console
+$ docker buildx build --push -t <user>/<image> \
+    --cache-to type=inline \
+    --cache-from type=registry,ref=<user>/<image>
+```
+
+## Further reading
+
+For an introduction to caching see [Optimizing builds with cache management](https://docs.docker.com/build/building/cache).
+
+For more information on the `inline` cache backend, see the [BuildKit README](https://github.com/moby/buildkit#inline-push-image-and-cache-together).