It has been upgraded to Python 3 and Django 2. This should cause no problems as Weblate itself supports both for quite some time, but if you were extending Weblate somehow, you might have to update these extensions to make them compatible.
The default cache backend is now redis. It will be required in future for some features, so you will have to switch at some point anyway. The memcached support is still there in case you want to stick with current setup.
Cron jobs have been integrated into the main container. So you no longer need to trigger them externally. This save quite some pain with offloaded indexing and another features which rely on regular execution.
Another important change is in logging - all logs are now go to the standard output, so you will get them by docker-compose logs and other Docker management commands. This will make debugging easier.