refactor: optimize Dockerfile build cache by separating dependency download

Dockerfile

@@ -3,13 +3,13 @@ FROM golang:1.21-alpine AS build
 
 WORKDIR /build
 
-# 直接复制所有源码
-COPY . .
+# 复制 go.mod 和 go.sum 以利用 Docker 缓存
+COPY go.mod go.sum ./
+RUN apk add --no-cache git && go mod download
 
-# 安装依赖并编译
-RUN apk add --no-cache git && \
-    go mod download && \
-    CGO_ENABLED=0 GOOS=linux go build -trimpath -ldflags="-s -w" -o /app/alist main.go
+# 复制剩余源码并编译
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -trimpath -ldflags="-s -w" -o /app/alist main.go
 
 # 生产阶段
 FROM alpine:3.19