build: add opt-out from default load behavior

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

build/build.go

@@ -343,11 +343,13 @@ func toSolveOpt(d driver.Driver, multiDriver bool, opt Options, dl dockerLoadCal
 		IsDefaultMobyDriver()
 	})
 
+	noDefaultLoad, _ := strconv.ParseBool(os.Getenv("BUILDX_NO_DEFAULT_LOAD"))
+
 	switch len(opt.Exports) {
 	case 1:
 		// valid
 	case 0:
-		if isDefaultMobyDriver {
+		if isDefaultMobyDriver && !noDefaultLoad {
 			// backwards compat for docker driver only:
 			// this ensures the build results in a docker image.
 			opt.Exports = []client.ExportEntry{{Type: "image", Attrs: map[string]string{}}}