Add support for multiple ENV key/values in one line

buildfile.go

func (b *buildFile) CmdEnv(args string) error {
	var env map[string]string

	if err := json.Unmarshal([]byte(args), &env); err != nil {
		env = []string{args}
	}
	if err :=  b.commit("", env, fm.Sprintf("ENV %v", env)); err != nil {
		return err
	}

	b.config.Env = env
	return nil
}