Fix CHANGE_SOURCE argument always false for nginx's Dockerfile (laradock#1786)

Author: ganl

DOCUMENTATION/content/documentation/index.md

@@ -1886,7 +1886,7 @@ This error sometimes happens because your Laravel application isn't running on t
 
 ## I get stuck when building nginx on `fetch http://mirrors.aliyun.com/alpine/v3.5/main/x86_64/APKINDEX.tar.gz`
 
-As stated on [#749](https://github.com/laradock/laradock/issues/749#issuecomment-293296687), removing the line `RUN sed -i 's/dl-cdn.alpinelinux.org/mirrors.aliyun.com/' /etc/apk/repositories` from `nginx/Dockerfile` solves the problem.		
+As stated on [#749](https://github.com/laradock/laradock/issues/749#issuecomment-419652646), Already fixed，just set `CHANGE_SOURCE` to false.		
 
 ## Custom composer repo packagist url and npm registry url
 

docker-compose.yml

@@ -193,6 +193,7 @@ services:
         args:
           - PHP_UPSTREAM_CONTAINER=${NGINX_PHP_UPSTREAM_CONTAINER}
           - PHP_UPSTREAM_PORT=${NGINX_PHP_UPSTREAM_PORT}
+          - CHANGE_SOURCE=${CHANGE_SOURCE}
       volumes:
         - ${APP_CODE_PATH_HOST}:${APP_CODE_PATH_CONTAINER}
         - ${NGINX_HOST_LOG_PATH}:/var/log/nginx