build: --set (#23512)

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## Description

addresses docker/buildx#872

Upstream PR here docker/buildx#3456

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- [ ] Technical review
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Author: aevesdocker

content/manuals/build/bake/overrides.md

@@ -212,6 +212,12 @@ $ docker buildx bake --set app.args.mybuildarg=bar --set app.platform=linux/arm6
 }
 ```
 
+> [!NOTE]
+>
+> `--set` is a repeatable flag. For array fields such as `tags`, repeat `--set` to provide multiple values or use the `+=` operator to append without replacing.
+> Array literal syntax like `--set target.tags=[a,b]` is not supported.
+> The `platform` field is a special case. It also accepts a comma-separated list.
+
 Pattern matching syntax defined in [https://golang.org/pkg/path/#Match](https://golang.org/pkg/path/#Match)
 is also supported: