Add exposing port in docker container (#6165)

Add port exposure for docker run command.

README.md

@@ -99,7 +99,7 @@ $ git clone https://github.com/parse-community/parse-server
 $ cd parse-server
 $ docker build --tag parse-server .
 $ docker run --name my-mongo -d mongo
-$ docker run --name my-parse-server --link my-mongo:mongo -d parse-server --appId APPLICATION_ID --masterKey MASTER_KEY --databaseURI mongodb://mongo/test
+$ docker run --name my-parse-server -p 1337:1337 --link my-mongo:mongo -d parse-server --appId APPLICATION_ID --masterKey MASTER_KEY --databaseURI mongodb://mongo/test
 ```
 
 You can use any arbitrary string as your application id and master key. These will be used by your clients to authenticate with the Parse Server.