Add Linux playground (#11605)

* Add Linux playground

* Update Online Java Playground link

* Update free-programming-playgrounds.md: Remove duplicate Java playground link

* Update free-programming-playgrounds.md: Remove duplicate Java playground link

more/free-programming-playgrounds.md

@@ -27,6 +27,7 @@
     * [AngularJS](#angularjs)
 * [Kotlin](#kotlin)
 * [Kubernetes](#kubernetes)
+* [Linux](#linux)
 * [Multiple Languages](#multiple-languages)
 * [.Net](#dotnet)
 * [Nim](#nim)
@@ -140,6 +141,7 @@
 
 ### Docker
 
+* [Online Docker Playground](https://labex.io/tutorials/docker-online-docker-playground-372912)
 * [Play with Docker](https://labs.play-with-docker.com)
 
 
@@ -211,6 +213,7 @@
 * [Java Compiler](https://www.codinguru.online/compiler/java)
 * [JDoodle - Online Java Compiler Advanced IDE](https://www.jdoodle.com/online-java-compiler-ide/)
 * [JDoodle - Online Java Compiler IDE](https://www.jdoodle.com/online-java-compiler/)
+* [Online Java Playground](https://labex.io/tutorials/java-online-java-playground-372914)
 * [OnlineGDB](https://www.onlinegdb.com/online_java_compiler)
 * [Programiz - Online Java Compiler](https://www.programiz.com/java-programming/online-compiler/)
 * [repl.it](https://repl.it) (_including a separate Java/Swing_)
@@ -255,6 +258,11 @@
 * [Play with Kubernetes](https://labs.play-with-k8s.com)
 
 
+### Linux
+
+* [Online Linux Terminal and Playground](https://labex.io/tutorials/linux-online-linux-playground-372915)
+
+
 ### Markdown
 
 * [Markdown Editor](https://www.codinguru.online/compiler/markdown)