provisioners/docker: allow images to be configured with images:

plugins/provisioners/docker/config.rb

@@ -12,6 +12,10 @@ module VagrantPlugins
         @version    = :latest
       end
 
+      def images=(images)
+        @images = Set.new(images)
+      end
+
       def pull_images(*images)
         @images += images.map(&:to_s)
       end

plugins/provisioners/docker/provisioner.rb

@@ -12,6 +12,7 @@ module VagrantPlugins
     class Provisioner < Vagrant.plugin("2", :provisioner)
       def initialize(machine, config, installer = nil, client = nil)
         super(machine, config)
+
         # TODO: Rename to installer / client (drop docker suffix)
         @installer = installer || DockerInstaller.new(@machine, config.version)
         @client    = client    || DockerClient.new(@machine)