It's stated as a "single container preview" - so the team is stating this was meant to be ephemeral.
Two paths (the team might know yet another way):
1. You can extract your data and then use a newer version, with a more permanent setup
You can get a working shell inside the running container with docker exec, you can find mysql credentials here and probably do a mysql dump.
To copy files out of the container and to your host, you can use docker cp.
2. You can try and upgrade the instance inside the container
Again, use docker exec to gain a shell inside the running container, then upgrade the Mattermost instance from inside the container.