put netbox restore database functionality inside container

[mmguero]

If you have a previous NetBox database backup (created with netbox-backup as described here) that you would like automatically restored on startup, that .gz file may be manually copied to the ./netbox/preload directory. Upon startup that file will be extracted and used to populate the NetBox database, taking priority over the other preload files. This process does not remove the .gz file from the directory upon restoring it; it will be restored again on subsequent restarts unless manually removed.

With the logic moved to inside the container, I may remove it from the control.py script completely and just trigger the internal container logic

first commit, mmguero-dev/Malcolm@1ceddc7