The Node-RED userDir is in /data in the Docker container, but you really should not be manually installing nodes into container on the command line as they are tracked by FlowFuse when installed via the UI so they can be re-installed if the container/stack needs to be restarted.
Docker containers use what is known as overlay filesystems which map on to the host system. If you have run out of disk space it is most likely because you have filled the host filesystem. You should check if it is full by running df -h.
You can normally increase the size of a EC2 instance file system by increasing the volume in the AWS console and then running the commands as root to expand the filesystem. e.g Expand the EBS root volume of your EC2 Linux instance | AWS re:Post
