Document Docker image cleanup and add docker-prune script.

Explain safe prune vs aggressive prune, compose down --rmi local, and when to run after rebuilds.

Co-authored-by: Cursor <cursoragent@cursor.com>

scripts/docker-prune.sh

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+#!/usr/bin/env bash
+# 清理 Docker 无用镜像与构建缓存
+# 用法：
+#   bash scripts/docker-prune.sh           # 安全：悬空镜像 + 构建缓存
+#   bash scripts/docker-prune.sh --all     # 额外删除未被任何容器使用的镜像
+
+set -euo pipefail
+
+APP_DIR="${APP_DIR:-/opt/zhimingge}"
+cd "$APP_DIR" 2>/dev/null || cd "$(dirname "$0")/.."
+
+echo "==> 当前磁盘占用"
+docker system df
+echo ""
+
+echo "==> 当前知命阁相关镜像"
+docker images --format "table {{.Repository}}\t{{.Tag}}\t{{.ID}}\t{{.Size}}\t{{.CreatedSince}}" \
+  | head -1
+docker images --format "table {{.Repository}}\t{{.Tag}}\t{{.ID}}\t{{.Size}}\t{{.CreatedSince}}" \
+  | grep -E "zhimingge|^REPOSITORY" || true
+echo ""
+
+echo "==> 删除悬空镜像（<none>，多为 rebuild 后遗留）..."
+docker image prune -f
+
+echo "==> 清理构建缓存..."
+docker builder prune -f
+
+if [[ "${1:-}" == "--all" ]]; then
+  echo ""
+  echo "==> 删除未被任何容器使用的镜像（不含正在运行的 zhimingge:latest）..."
+  docker image prune -a -f
+fi
+
+echo ""
+echo "==> 清理后磁盘占用"
+docker system df
+echo ""
+echo "完成。若仍占用较大，可查看：docker images -a"