Update allowed_images fully-open example

When using private Docker registries
(e.g. docker-reg.example.com/name/image),
you need to include `"*/*/*"` in `allowed_images`.

doc/docker/using_docker_images.md

@@ -154,7 +154,7 @@ If you are courageous enough, you can make it fully open and accept everything:
 ```
 [runners.docker]
   image = "ruby:2.1"
-  allowed_images = ["*", "*/*"]
+  allowed_images = ["*", "*/*", "*/*/*"]
   allowed_services = ["*", "*/*"]
 ```
@@ -200,4 +200,4 @@ The build_script is piped using STDIN to bash interpreter which executes the bui
 docker rm -f -v build service-mysql service-postgres
 ```
 This will forcefully (the `-f` switch) remove build container and service containers 
-and all volumes (the `-v` switch) that were created with the container creation.
+and all volumes (the `-v` switch) that were created with the container creation.
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