Adds host flag to docs

Author: cmoog

doc/CONTRIBUTING.md

@@ -17,7 +17,7 @@ To develop inside of an isolated docker container:
 
 root@12345:/code-server# yarn
 root@12345:/code-server# yarn vscode
-root@12345:/code-server# yarn watch
+root@12345:/code-server# yarn watch --host=0.0.0.0
 ```
 
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