Support BuildArgs for arg command

pkg/dockerfile/buildargs.go

@@ -0,0 +1,35 @@
+/*
+Copyright 2018 Google LLC
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package dockerfile
+
+import (
+	d "github.com/docker/docker/builder/dockerfile"
+	"strings"
+)
+
+func NewBuildArgs(args []string) *d.BuildArgs {
+	argsFromOptions := make(map[string]*string)
+	for _, a := range args {
+		s := strings.Split(a, "=")
+		if len(s) == 1 {
+			argsFromOptions[s[0]] = nil
+		} else {
+			argsFromOptions[s[0]] = &s[1]
+		}
+	}
+	return d.NewBuildArgs(argsFromOptions)
+}