provisioners/docker: output for each image pulling

plugins/provisioners/docker/docker_client.rb

@@ -10,6 +10,7 @@ module VagrantPlugins
       def pull_images(*images)
         @machine.communicate.tap do |comm|
           images.each do |image|
+            @machine.ui.info(I18n.t("vagrant.docker_pulling_single", name: image))
             comm.sudo("docker images | grep -q #{image} || docker pull #{image}")
           end
         end

templates/locales/en.yml

@@ -47,6 +47,8 @@ en:
       Installing Docker onto machine...
     docker_pulling_images:
       Pulling Docker images...
+    docker_pulling_single: |-
+      -- Image: %{name}
     docker_running: |-
       -- Container: %{name}
     docker_starting_containers: