Added details break into container for plugin sandbox

Added details for how to break into the container and run the CakePHP
database command to the plugin sandbox instructions.

docs/simple-development-plugin-mariadb.md

@@ -188,6 +188,18 @@ in the [COmanage Registry Technical Manual](https://spaces.at.internet2.edu/x/Og
 See also the [COmanage Developer Manual](https://spaces.at.internet2.edu/x/FYDVCQ), and
 specifically the wiki page on [COmanage Coding Style](https://spaces.at.internet2.edu/x/l6_KAQ).
 
+* If your plugin adds new tables to the database schema you will need to break into the running
+COmanage Registry container to run the CakePHP `database` command. To break into the container
+and run the `database` command:
+
+```
+docker exec -it comanage_comanage-registry_1 /bin/bash
+cd app
+./Console/cake database
+```
+
+When you are finished enter `exit` to end the bash shell process and exit the container.
+
 ## Stopping and removing services
 
 * To stop the services: