feat: allow setting platform for scratch image

BuildKit refers to platform set in the current LLB node for determining whether or not executing commands should use QEMU. This makes some build flows working fine without BuildKit to fail with it. This is of particular importance for cross-compilation cases.

Make scratch image a no exception and allow platform to be set, therefore determining the architecture of binaries planned to be ran (e.g. if they are COPYed from contexts)

frontend/dockerfile/dockerfile2llb/convert.go

@@ -468,9 +468,12 @@ func toDispatchState(ctx context.Context, dt []byte, opt ConvertOpt) (*dispatchS
 			if d.base == nil && !d.dispatched && !d.resolved {
 				d.resolved = reachable // avoid re-resolving if called again after onbuild
 				if d.stage.BaseName == emptyImageName {
-					d.state = llb.Scratch()
-					d.image = emptyImage(platformOpt.targetPlatform)
-					d.platform = &platformOpt.targetPlatform
+					platform := d.platform
+					if platform == nil {
+						platform = &platformOpt.targetPlatform
+					}
+					d.state = llb.Scratch().Platform(*platform)
+					d.image = emptyImage(*platform)
 					if d.unregistered {
 						d.dispatched = true
 					}
@@ -511,7 +514,7 @@ func toDispatchState(ctx context.Context, dt []byte, opt ConvertOpt) (*dispatchS
 							if img != nil {
 								d.image = *img
 							} else {
-								d.image = emptyImage(platformOpt.targetPlatform)
+								d.image = emptyImage(*platform)
 							}
 							d.state = st.Platform(*platform)
 							d.platform = platform