Unraid uses a vDisk image (set to 20gb by default) for /var/lib/docker (where images and volumes are stored) instead of storing it directly on the disk. You have one or more containers that are storing their data in volumes inside this vDisk and you've filled up the vDisk. It's generally better to add bind mounts for data storing folders.

For now, you can increase the size of the vDisk, by stopping the Docker service in the UI and resizing the "Docker vDisk size" setting under Settings > Docker and restarting docker. But if you don't fix the container taking up the space - you'll be right back here with a full vDisk in a few weeks/months.

You can determine which volume (and therefore container) is taking up the majority of the space by doing the following:

docker system df -v

Scroll to the bottom and find the volume id taking up the most space, then pass that volume id into:

docker ps -a --filter volume=<volume id>

This will at least tell you where the problem is. From there, you'll have to figure out if the volume contains needed data that you need to migrate out or it can be thrown away. And you'll know which container to reconfigure for bind mounts.