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

commands/build.go

@@ -84,8 +84,8 @@ func runBuild(dockerCli command.Cli, in buildOptions) error {
 			InStream:       os.Stdin,
 		},
 		Tags:        in.tags,
-		Labels:      listToMap(in.labels),
-		BuildArgs:   listToMap(in.buildArgs),
+		Labels:      listToMap(in.labels, false),
+		BuildArgs:   listToMap(in.buildArgs, true),
 		Pull:        in.pull,
 		NoCache:     in.noCache,
 		Target:      in.target,
@@ -282,12 +282,16 @@ func commonFlags(options *commonOptions, flags *pflag.FlagSet) {
 	flags.BoolVar(&options.pull, "pull", false, "Always attempt to pull a newer version of the image")
 }
 
-func listToMap(values []string) map[string]string {
+func listToMap(values []string, defaultEnv bool) map[string]string {
 	result := make(map[string]string, len(values))
 	for _, value := range values {
 		kv := strings.SplitN(value, "=", 2)
 		if len(kv) == 1 {
+			if defaultEnv {
+				result[kv[0]] = os.Getenv(kv[0])
+			} else {
 				result[kv[0]] = ""
+			}
 		} else {
 			result[kv[0]] = kv[1]
 		}