docs(helm): add image_registry docs

Author: duanhongyi

src/installing-workflow/index.md

@@ -63,6 +63,35 @@ $ helm install --namespace drycc \
     drycc/workflow
 ```
 
+By default, registry uses docker hub, at present, we support three registries. They are: 
+
+* redhat quay `quay.io`
+* docker hub `docker.io`
+* aliyun cr `registry.cn-hangzhou.aliyuncs.com`
+
+If you want to change it, set the variable when using helm.
+
+```
+$ helm install --namespace drycc \
+    --set builder.image_registry=quay.io/ \
+    --set slugbuilder.image_registry=quay.io/ \
+    --set imagebuilder.image_registry=quay.io/ \
+    --set controller.image_registry=quay.io/ \
+    --set slugrunner.image_registry=quay.io/ \
+    --set database.image_registry=quay.io/ \
+    --set fluentd.image_registry=quay.io/ \
+    --set redis.image_registry=quay.io/ \
+    --set influxdb.image_registry=quay.io/ \
+    --set logger.image_registry=quay.io/ \
+    --set minio.image_registry=quay.io/ \
+    --set monitor.image_registry=quay.io/ \
+    --set nsqd.image_registry=quay.io/ \
+    --set registry.image_registry=quay.io/ \
+    --set registry-proxy.image_registry=quay.io/ \
+    --set global.platform_domain=drycc.cc \
+    drycc/workflow
+```
+
 Helm will install a variety of Kubernetes resources in the `drycc` namespace.
 Wait for the pods that Helm launched to be ready. Monitor their status by running: