Update README.md

Author: SamuelTallet

docs/README.md

@@ -16,7 +16,7 @@ Additional features: Export documents to JSON. Import documents from JSON. Manag
 ## Installation
 
 ### Docker (PHP built-in server)
-1. Execute `docker run --rm -p 5000:5000 samueltallet/mongodb-php-gui`<br>
+1. Run `docker pull samueltallet/mongodb-php-gui && docker run --rm -p 5000:5000 samueltallet/mongodb-php-gui`<br>
 2. Open your browser at this address: http://127.0.0.1:5000/ to access GUI.<br>
 3. If your MongoDB is running on localhost, use 172.17.0.1 as host to login.