As you can see here How to update software inside a docker container? - Stack Overflow the best practice is to not update the software running inside the container, but to make a new container by pulling the new image.
Make sure you mapped the UserData folder in the container to a local folder, so that you won’t lose your data.
First of all stop the container. Then delete the stopped container and your local openbullet/openbullet2:latest image. If that’s the only image you have you can also run this command
docker system prune -a
Then pull the new image and run a new container with the same command you used to start your current one.
The only thing you will lose are themes.
Ruri