doc: add port forwarding in `docke run` command

README.md

@@ -33,7 +33,7 @@ Here is what you need to build and run a code elevator session :
 
 ## Simple run with Docker
 
-    $ docker run registry.gitlab.inria.fr/sed-lille/code-contest/code-elevator
+    $ docker run -p 8080:8080 registry.gitlab.inria.fr/sed-lille/code-contest/code-elevator[:latest|master|<version>]
 
 ### Production
 To run it in production we add more options: