fix: emit BUILDKIT_MULTI_PLATFORM=1

Image Factory assumes registry images are multi-architecture (e.g. the
top-level object is an image index). However builx will push a raw image
manifest if it only has one image to push (single architecture, no
provenance or attestaton). Passing `BUILDKIT_MULTI_PLATFORM=1` ensures
an image index is always pushed.

Signed-off-by: Jean-Francois Roy <jf@devklog.net>
Signed-off-by: Andrey Smirnov <andrey.smirnov@siderolabs.com>

Makefile

@@ -1,6 +1,6 @@
 # THIS FILE WAS AUTOMATICALLY GENERATED, PLEASE DO NOT EDIT.
 #
-# Generated on 2024-10-22T09:07:30Z by kres 34e72ac-dirty.
+# Generated on 2024-11-05T09:30:48Z by kres 38fd548.
 
 # common variables
 
@@ -46,6 +46,7 @@ COMMON_ARGS += --provenance=false
 COMMON_ARGS += --progress=$(PROGRESS)
 COMMON_ARGS += --platform=$(PLATFORM)
 COMMON_ARGS += --push=$(PUSH)
+COMMON_ARGS += --build-arg=BUILDKIT_MULTI_PLATFORM=1
 COMMON_ARGS += --build-arg=ARTIFACTS="$(ARTIFACTS)"
 COMMON_ARGS += --build-arg=SHA="$(SHA)"
 COMMON_ARGS += --build-arg=TAG="$(TAG)"

internal/project/common/docker.go

@@ -117,7 +117,8 @@ func (docker *Docker) CompileMakefile(output *makefile.Output) error {
 		Push("--provenance=false").
 		Push("--progress=$(PROGRESS)").
 		Push("--platform=$(PLATFORM)").
-		Push("--push=$(PUSH)")
+		Push("--push=$(PUSH)").
+		Push("--build-arg=BUILDKIT_MULTI_PLATFORM=1")
 
 	for _, arg := range docker.meta.BuildArgs {
 		buildArgs.Push(fmt.Sprintf("--build-arg=%s=\"$(%s)\"", arg, arg))

internal/project/pkgfile/build.go

@@ -83,7 +83,8 @@ func (pkgfile *Build) CompileMakefile(output *makefile.Output) error {
 		Push("--provenance=false").
 		Push("--progress=$(PROGRESS)").
 		Push("--platform=$(PLATFORM)").
-		Push("--build-arg=SOURCE_DATE_EPOCH=$(SOURCE_DATE_EPOCH)")
+		Push("--build-arg=SOURCE_DATE_EPOCH=$(SOURCE_DATE_EPOCH)").
+		Push("--build-arg=BUILDKIT_MULTI_PLATFORM=1")
 
 	for _, arg := range pkgfile.ExtraBuildArgs {
 		buildArgs.Push(fmt.Sprintf("--build-arg=%s=\"$(%s)\"", arg, arg))