Rename exercises dir

Name it instead "topics" so it won't be
strange if some topics included "exercises" directory.

topics/containers/image_layers.md

@@ -0,0 +1,23 @@
+## Layer by Layer
+
+### Objective
+
+Learn about image layers
+
+### Requirements
+
+Make sure Docker is installed on your system and the service is started
+
+```
+# Fedora/RHEL/CentOS
+rpm -qa | grep docker
+systemctl status docker
+```
+
+### Instructions
+
+1. Write a Dockefile. Any Dockefile! :) (just make sure it's a valid one)
+2. Build an image using the Dockerfile you've wrote
+3. Which of the instructions you've used, created new layers and which added image metadata?
+4. What ways are there to confirm your answer to the last question?
+5. Can you reduce the size of the image you've created?