fix: add persistent volume data directory for volume server

- Add -dir=/data flag to volume server command
- Mount Docker volume seaweedfs-volume-data to /data
- Ensures volume server has persistent storage for volume files
- Fixes issue where volume server couldn't create writable volumes
- Volume data persists across container restarts during tests

test/java/spark/docker-compose.yml

@@ -27,7 +27,9 @@ services:
     ports:
       - "8080:8080"
       - "18080:18080"
-    command: "volume -mserver=seaweedfs-master:9333 -ip=127.0.0.1 -ip.bind=0.0.0.0 -port=8080 -port.grpc=18080 -publicUrl=127.0.0.1:8080 -max=0 -preStopSeconds=1"
+    command: "volume -mserver=seaweedfs-master:9333 -ip=127.0.0.1 -ip.bind=0.0.0.0 -port=8080 -port.grpc=18080 -publicUrl=127.0.0.1:8080 -max=0 -dir=/data -preStopSeconds=1"
+    volumes:
+      - seaweedfs-volume-data:/data
     depends_on:
       seaweedfs-master:
         condition: service_healthy
@@ -86,3 +88,6 @@ networks:
   seaweedfs-spark:
     driver: bridge
 
+volumes:
+  seaweedfs-volume-data:
+