chore(deploy): prune stale build tags

Author: ieduer

README.md

@@ -301,7 +301,7 @@ RUNTIME_ROOT=/root/cross-subject-knowledge ./scripts/deploy_vps.sh
 4.  在 VPS 上先构建新镜像，再停旧容器，避免“构建失败直接打挂线上”。
 5.  新容器通过 `/api/health` 健康检查后才算部署成功；失败则自动回滚到上一镜像。
 6.  运行时模型缓存保存在宿主机 `state/cache/`，避免每次发版都把 Hugging Face 缓存烘进镜像。
-7.  部署完成后自动清理悬空镜像，并只保留最近几份 `pre-*` 回滚镜像。
+7.  部署完成后自动清理悬空镜像，只保留最近几份 `pre-*` 回滚镜像，并删除历史 `build-*` tag。
 
 ### 3. 服务器 (VPS) 迁移指南
 由于大头数据 (4GB+ 图片) 都在云端 CDN，如果未来需要更换服务器提供商，迁移将极其简单轻量：

docs/runtime_operations_overview.md

@@ -58,7 +58,7 @@ Current production deploy is:
 4. Host model cache is reused or warmed before cutover
 5. Old container is replaced only after image build succeeds
 6. `/api/health` must pass or the script rolls back to the previous image
-7. Dangling images are pruned; only recent rollback tags are retained
+7. Dangling images are pruned; only recent rollback tags are retained and old `build-*` tags are removed
 
 ## Operational watchpoints
 

scripts/deploy_vps.sh

@@ -136,6 +136,14 @@ if [ -n "${old_pre_tags}" ]; then
   done <<< "${old_pre_tags}"
 fi
 
+old_build_tags="$(docker images --format '{{.Repository}}:{{.Tag}}' | grep '^textbook-knowledge:build-' | grep -v "^${build_tag}$" || true)"
+if [ -n "${old_build_tags}" ]; then
+  while IFS= read -r image_ref; do
+    [ -n "${image_ref}" ] || continue
+    docker rmi "${image_ref}" >/dev/null 2>&1 || true
+  done <<< "${old_build_tags}"
+fi
+
 echo "=== disk after deploy ==="
 df -h /
 docker system df