fix: Docker entrypoint 以 root 运行再降权，彻底解决 volume 权限问题

去掉 USER appuser，entrypoint 以 root 身份运行，先 chown 修复
volume 挂载目录的权限，再通过 su 降权到 appuser 执行应用。

docker-entrypoint.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+set -e
+
+# Fix permissions on volume-mounted directories (runs as root)
+chown -R appuser:appgroup /app/db /app/logs /app/resources 2>/dev/null || true
+
+# Run Java directly - entrypoint is PID 1, exec makes Java PID 1
+# Docker SIGTERM goes directly to Java, triggering ShutdownHook
+exec java -Xmx${JVM_XMX:-512M} ${JVM_OPTS} -jar /app/netdisk-fast-download.jar