You can join the public test network with such amount of space.
You need to disable storage monitor either in the config.yaml by parameter
storage2.monitor.minimum-disk-space: 0B
or in the docker run command by specifying option --storage2.monitor.minimum-disk-space=0B after the docker image name, i.e.
docker run ... storjlabs/storagenode --storage2.monitor.minimum-disk-space=0B