Hi @functioneer
It seems like you're running the preview version and without storing the data in a separate volume. So all the data is inside. If you remove it the container you'll lose all of your data.
If this is indeed a preview and the data doesn't matter then proceed here:
That's Docker telling you that a container for that image already exists. Do this to remove the container, so that it creates the new container with the new image when you run the specified command (so run this first):
docker rm mattermost-preview
Now, if you have data, you need to rescue it from inside the container and save it somewhere else. Containers store their volume data inside /var folder (a really long folder name).
See here for more details about volumes
There's an explanation of the command docker inspect <name of your container> which can tell you where your data is located. So my advice will be to use that, get the volume where the data is being saved, copy it to another folder, then run the preview image (create a separate container, with a different name) but this time mount the volume to that folder (where you copied the files earlier) and see if your data is there.
Let me know how it goes!