debug: add DNS verification and disable Java DNS caching

Troubleshooting 'seaweedfs-volume: Temporary failure in name resolution':

docker-compose.yml changes:
- Add MAVEN_OPTS to disable Java DNS caching (ttl=0)
  Java caches DNS lookups which can cause stale results
- Add ping tests before mvn test to verify DNS resolution
  Tests: ping -c 1 seaweedfs-volume && ping -c 1 seaweedfs-filer
- This will show if DNS works before tests run

workflow changes:
- List Docker networks before running tests
- Shows network configuration for debugging
- Helps verify spark-tests joins correct network

If ping succeeds but tests fail, it's a Java/Maven DNS issue.
If ping fails, it's a Docker networking configuration issue.

Note: Previous test failures may be from old code before Docker networking fix.

test/java/spark/docker-compose.yml

@@ -79,7 +79,9 @@ services:
       - SEAWEEDFS_FILER_PORT=8888
       - SEAWEEDFS_FILER_GRPC_PORT=18888
       - HADOOP_HOME=/tmp
-    command: sh -c "sleep 30 && mvn test"
+      # Disable Java DNS caching to ensure fresh DNS lookups
+      - MAVEN_OPTS=-Dsun.net.inetaddr.ttl=0 -Dnetworkaddress.cache.ttl=0
+    command: sh -c "sleep 30 && ping -c 1 seaweedfs-volume && ping -c 1 seaweedfs-filer && mvn test"
     depends_on:
       seaweedfs-filer:
         condition: service_healthy