allow flag --target in build process

cmd/drone-docker/main.go

@@ -146,6 +146,11 @@ func main() {
 			Usage:  "build args",
 			EnvVar: "PLUGIN_BUILD_ARGS_FROM_ENV",
 		},
+		cli.StringFlag{
+			Name:   "target",
+			Usage:  "build target",
+			EnvVar: "PLUGIN_BUILD_TARGET",
+		},
 		cli.BoolFlag{
 			Name:   "squash",
 			Usage:  "squash the layers at build time",
@@ -227,6 +232,7 @@ func run(c *cli.Context) error {
 			Tags:        c.StringSlice("tags"),
 			Args:        c.StringSlice("args"),
 			ArgsEnv:     c.StringSlice("args-from-env"),
+			Target:      c.String("target"),
 			Squash:      c.Bool("squash"),
 			Pull:        c.BoolT("pull-image"),
 			Compress:    c.Bool("compress"),

docker.go

@@ -44,6 +44,7 @@ type (
 		Tags        []string // Docker build tags
 		Args        []string // Docker build args
 		ArgsEnv     []string // Docker build args from env
+		Target      string   // Docker build target
 		Squash      bool     // Docker build squash
 		Pull        bool     // Docker build pull
 		Compress    bool     // Docker build compress
@@ -204,6 +205,9 @@ func commandBuild(build Build) *exec.Cmd {
 	for _, arg := range build.Args {
 		args = append(args, "--build-arg", arg)
 	}
+	if build.Target != "" {
+		args = append(args, "--target", build.Target)
+	}
 
 	labelSchema := []string{
 		fmt.Sprintf("build-date=%s", time.Now().Format(time.RFC3339)),