Rename exercises dir

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

topics/containers/containerized_db.md

@@ -0,0 +1,9 @@
+## Containerized DB
+
+1. Run a container with a database of any type of you prefer (MySql, PostgreSQL, Mongo, etc.)
+2. Verify the container is running
+3. Access the container and create a new table (or collection, depends on which DB type you chose) for students
+4. Insert a row (or document) of a student
+5. Verify the row/document was added
+
+Click [here for the solution](solutions/containerized_db.md)