Add Docker and Kubernetes courses and adjust sub headings (#3889)

* Add course for learning Docker

* Add course for learning Kubernetes

* Fix headings for javascript section

* Add name of platform

* Fix alphabetical order

* Fix white space

free-courses-en.md

-### Index
+### Index
 
 * [Algorithms & Data Structures](#algorithms--data-structures)
 * [Android](#android)
@@ -30,6 +30,7 @@
   * [Redux](#redux)
   * [Sails.js](#sailsjs)
   * [Vue.js](#vuejs)
+* [Kubernetes](#kubernetes)
 * [Linux](#linux)
 * [Lua](#lua)
 * [Machine Learning](#machine-learning)
@@ -183,6 +184,7 @@
 
 * [Docker Tutorial](https://www.tutorialspoint.com/docker/index.htm) - TutorialsPoint
 * [Docker Tutorial for Beginners](https://www.youtube.com/watch?v=fqMOX6JJhGo) - FreeCodeCamp
+* [Learn Docker, Container Runtimes, Builders and Registries](https://www.katacoda.com/courses/container-runtimes) - Katacoda
 
 
 ### Game Development
@@ -299,16 +301,21 @@
 * [Getting Started with Redux](https://egghead.io/series/getting-started-with-redux)
 
 
-### Sails.Js
+#### Sails.Js
 
 * [Develop web apps in free-programming-playgrounds.md.js and Sails.js](https://courses.platzi.com/courses/develop-apps-sails-js/)
 
 
-### Vue.js
+#### Vue.js
 
 * [Intro to Vue 3](https://www.vuemastery.com/courses/intro-to-vue-3/intro-to-vue3)
 
 
+### Kubernetes
+
+* [Learn Kubernetes](https://www.katacoda.com/courses/kubernetes) - Katacoda
+
+
 ### Linux
 
 * [Introduction To Linux](https://www.edx.org/course/introduction-to-linux/) - The Linux Foundation (edx)