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docs: document cache-from 

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
Signed-off-by: Tibor Vass <tibor@docker.com>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 73cd257d0f)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
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Tonis Tiigi 2019-05-15 11:12:41 -07:00 committed by Sebastiaan van Stijn
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docs/reference/commandline/build.md
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@ -489,6 +489,46 @@ FROM alpine AS production-env
$ docker build -t mybuildimage --target build-env .
```
### Specifying external cache sources
In addition to local build cache, the builder can reuse the cache generated from
previous builds with the `--cache-from` flag pointing to an image in the registry.
To use an image as a cache source, cache metadata needs to be written into the
image on creation. This can be done by setting `--build-arg BUILDKIT_INLINE_CACHE=1`
when building the image. After that, the built image can be used as a cache source
for subsequent builds.
Upon importing the cache, the builder will only pull the JSON metadata from the
registry and determine possible cache hits based on that information. If there
is a cache hit, the matched layers are pulled into the local environment.
In addition to images, the cache can also be pulled from special cache manifests
generated by [`buildx`](https://github.com/docker/buildx) or the BuildKit CLI
(`buildctl`). These manifests (when built with the `type=registry` and `mode=max`
options) allow pulling layer data for intermediate stages in multi-stage builds.
The following example builds an image with inline-cache metadata and pushes it
to a registry, then uses the image as a cache source on another machine:
```bash
$ docker build -t myname/myapp --build-arg BUILDKIT_INLINE_CACHE=1 .
$ docker push myname/myapp
```
After pushing the image, the image is used as cache source on another machine.
BuildKit automatically pulls the image from the registry if needed.
```bash
# on another machine
$ docker build --cache-from myname/myapp .
```
> **Note**: This feature requires the BuildKit backend. You can either
> [enable BuildKit](../builder.md#buildkit) or use the [buildx](https://github.com/docker/buildx)
> plugin. The previous builder has limited support for reusing cache from
> pre-pulled images.
### Squash an image's layers (--squash) (experimental)
#### Overview