gcloud registry auth step for docker build

nowaday, the docker image build from gcloud registry, so you need auth the configure and login the gcloud first to build the docker image.

DOCUMENTATION/content/getting-started/index.md

@@ -8,6 +8,7 @@ weight: 2
 
 - [Git](https://git-scm.com/downloads)
 - [Docker](https://www.docker.com/products/docker/) `>= 17.12`
+- [gcloud](https://cloud.google.com/sdk/install) 
 
 
 
@@ -24,6 +25,17 @@ Choose the setup the best suits your needs.
  	- [A.2) Don't have a PHP project yet](#A2)
 - [B) Setup for Multiple Projects](#B)
 
+### Setup gcloud for docker registry
+
+```
+gcloud auth configure-docker
+```
+
+and login to gcloud for use the registry and auth the permission.
+
+```
+gcloud auth login
+```
 
 <a name="A"></a>
 ### A) Setup for Single Project