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docs: note ARG defaults override automatic TARGETPLATFORM #25847

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12 changes: 12 additions & 0 deletions content/manuals/build/building/multi-platform.md
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Expand Up @@ -226,6 +226,18 @@ FROM alpine
COPY --from=build /log /log
```

Declare these automatic arguments with no default. A default such as
`ARG TARGETPLATFORM=linux/amd64` is treated as a user-set value, so it wins
over `--platform`. If you need a fallback for builders that do not set the
automatic arguments, apply it later:

```dockerfile
ARG TARGETPLATFORM
ENV TARGETPLATFORM=${TARGETPLATFORM:-linux/amd64}
```

The same applies to `TARGETOS`, `TARGETARCH`, and `TARGETVARIANT`.

## Examples

Here are some examples of multi-platform builds:
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