add build_args_from_env parameter

cmd/drone-docker/main.go

@@ -126,6 +126,11 @@ func main() {
 			Usage:  "build args",
 			EnvVar: "PLUGIN_BUILD_ARGS",
 		},
+		cli.StringSliceFlag{
+			Name:   "args-from-env",
+			Usage:  "build args",
+			EnvVar: "PLUGIN_BUILD_ARGS_FROM_ENV",
+		},
 		cli.BoolFlag{
 			Name:   "squash",
 			Usage:  "squash the layers at build time",
@@ -195,6 +200,7 @@ func run(c *cli.Context) error {
 			Context:     c.String("context"),
 			Tags:        c.StringSlice("tags"),
 			Args:        c.StringSlice("args"),
+			ArgsEnv:     c.StringSlice("args-from-env"),
 			Squash:      c.Bool("squash"),
 			Pull:        c.BoolT("pull-image"),
 			Compress:    c.Bool("compress"),

docker.go

@@ -43,6 +43,7 @@ type (
 		Context     string   // Docker build context
 		Tags        []string // Docker build tags
 		Args        []string // Docker build args
+		ArgsEnv     []string // Docker build args from env
 		Squash      bool     // Docker build squash
 		Pull        bool     // Docker build pull
 		Compress    bool     // Docker build compress
@@ -194,6 +195,9 @@ func commandBuild(build Build) *exec.Cmd {
 	if build.Pull {
 		args = append(args, "--pull=true")
 	}
+	for _, arg := range build.ArgsEnv {
+		addProxyValue(&build, arg)
+	}
 	for _, arg := range build.Args {
 		args = append(args, "--build-arg", arg)
 	}