build: load default build args from env

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

bake/compose.go

@@ -2,6 +2,7 @@ package bake
 
 import (
 	"fmt"
+	"os"
 	"reflect"
 
 	"github.com/docker/cli/cli/compose/loader"
@@ -86,6 +87,8 @@ func toMap(in composetypes.MappingWithEquals) map[string]string {
 	for k, v := range in {
 		if v != nil {
 			m[k] = *v
+		} else {
+			m[k] = os.Getenv(k)
 		}
 	}
 	return m