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docs: document from platform 

Co-Authored-By: Tibor Vass <tiborvass@users.noreply.github.com>
Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
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Tonis Tiigi 2019-07-12 16:00:57 -07:00 committed by Sebastiaan van Stijn
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17
docs/reference/builder.md
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@ -558,15 +558,15 @@ the first pattern, followed by one or more `!` exception patterns.
## FROM
FROM <image> [AS <name>]
FROM [--platform=<platform>] <image> [AS <name>]
Or
FROM <image>[:<tag>] [AS <name>]
FROM [--platform=<platform>] <image>[:<tag>] [AS <name>]
Or
FROM <image>[@<digest>] [AS <name>]
FROM [--platform=<platform>] <image>[@<digest>] [AS <name>]
The `FROM` instruction initializes a new build stage and sets the
[*Base Image*](glossary.md#base-image) for subsequent instructions. As such, a
@ -576,21 +576,26 @@ the [*Public Repositories*](https://docs.docker.com/engine/tutorials/dockerrepos
- `ARG` is the only instruction that may precede `FROM` in the `Dockerfile`.
See [Understand how ARG and FROM interact](#understand-how-arg-and-from-interact).
- `FROM` can appear multiple times within a single `Dockerfile` to
create multiple images or use one build stage as a dependency for another.
Simply make a note of the last image ID output by the commit before each new
`FROM` instruction. Each `FROM` instruction clears any state created by previous
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.
- 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.
The optional `--platform` flag can be used to specify the platform of the image
in case `FROM` references a multi-platform image. For example, `linux/amd64`,
`linux/arm64`, or `windows/amd64`. By default, the target platform of the build
request is used. Global build arguments can be used in the value of this flag,
for example [automatic platform ARGs](#automatic-platform-args-in-the-global-scope)
allow you to force a stage to native build platform (`--platform=$BUILDPLATFORM`),
and use it to cross-compile to the target platform inside the stage.
### Understand how ARG and FROM interact
`FROM` instructions support variables that are declared by any `ARG`