fix(content): harden Compose credential examples

11_compose/11.8_wordpress.md

@@ -157,7 +157,7 @@ $ docker compose logs -f
 ```bash
 ## 导出 SQL
 
-$ docker exec wordpress_db mysqldump -u wordpress -pwordpress wordpress > backup.sql
+$ docker compose exec -T db sh -c 'tmp=$(mktemp) && printf "[client]\nuser=wordpress\npassword=%s\n" "$(cat /run/secrets/db_password)" > "$tmp" && mysqldump --defaults-extra-file="$tmp" wordpress; rc=$?; rm -f "$tmp"; exit "$rc"' > backup.sql
 ```
 或者添加一个自动备份容器：
 
@@ -171,8 +171,11 @@ $ docker exec wordpress_db mysqldump -u wordpress -pwordpress wordpress > backup
       - DB_HOST=db
       - DB_NAME=wordpress
       - DB_USER=wordpress
-      - DB_PASS=${DB_PASSWORD}
+      # 选用支持从文件读取密码的备份镜像，或用自定义 entrypoint 从 secret 文件注入。
+      - DB_PASS_FILE=/run/secrets/db_password
       - DB_DUMP_FREQ=1440 # 每天备份一次
+    secrets:
+      - db_password
     depends_on:
       - db
     networks: