Update devcontainer.json

Author: Chuxel

containers/docker-existing-dockerfile/.devcontainer/devcontainer.json

@@ -7,6 +7,9 @@
 
     // "Use the optional 'appPort' property to expose ports not already in your Dockerfile"
     "appPort": [],
+    
+    // "Add any extensions you want auto-installed here."
+    "extensions": [],
 
     // "The optional 'runArgs' property can be used to specify Docker CLI arguments to use when the container."
     // "is started. If you install the Docker CE CLI in your container, the runArgs list below will lets you interact"