Rename exercises dir

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

topics/containers/containerized_web_server.md

@@ -0,0 +1,8 @@
+# Containerized Web Server
+
+1. Run a containerized web server in the background and bind its port (8080) to a local port
+2. Verify the port (8080) is bound
+3. Reach the webserver from your local host
+4. Now run the same web application but bound it to the local port 8080
+
+Click [here for the solution](solutions/containerized_web_server.md)