Updated instructions how to start a container

00_GettingStarted/README.md

@@ -114,7 +114,7 @@ Follow the instructions to download the repository to your local machine and sta
         docker ps --all
         ```
         The terminal output will look the following:
-        ![Terminal output for *docker ps --all*](images/docker_ps_all_terminal.png)
+        ![Terminal output for *docker ps --all*](images/docker_ps_all.gif)
         There will be information like ID of the container, name of the image the container was build from etc.
 8. To connect to the ***i2r-ros-1*** container, the container we will use to work with `ROS`, execute the following command from terminal:
 
@@ -125,8 +125,20 @@ Follow the instructions to download the repository to your local machine and sta
     ![GIF: Terminal output for *docker exec -it i2r-ros-1 bash*](images/docker_exec_rosContainer.gif)
 
     This starts a ***bash* session** in the running *i2r-ros-1* container. ***Bash*** (Bourne-again shell) is a Unix-shell that interacts as a **human-machine interface** with an environment to allow line by line text **in-** and **output**.
+9. Now you can test if the *i2r-ros-1* container you started has `ROS` installed and working, by launching ***roscore***. The `ROS` specific terminal command *roscore* launches a collection of *nodes* that are essential to work with a `ROS`-based system. You will learn more about the basics of `ROS` in the [**Introduction to ROS**](../02_IntroductionToROS/) chapter.
+
+    ```bash
+    roscore
+    ```
+
+    ![GIF: Terminal output for *roscore*](images/roscore.gif)
+
+    If that worked out, you can again close the the started *ROS Master* by typing *`Ctrl` + `C`*. That shuts down the *Master* node. To leave the *bash* session inside of the *i2r-ros-1 container* just type *`Ctrl` + `D`*.
+
+    ![GIF: Terminal output kill master and leave container session.](images/kill_master_leave_container.gif)
 
 ## In case of problems
+
 If you run into any issues during the installation process of these softwares, you are welcome to ask me ([Contact Data](https://www.gut.rwth-aachen.de/cms/Geotechnik/Das-Institut/Team/~peawz/Jan-Holzheim/lidx/1/), via mail, phone or in presence).
 
 (Remark: Add notice to Tutorials)