fix: adjust Dockerfile build process to handle dependency download separately

Dockerfile

@@ -3,11 +3,13 @@ FROM golang:1.21-alpine AS build
 
 WORKDIR /build
 
-COPY . .
+# 复制源码
+COPY go.mod go.sum ./
+RUN apk add --no-cache git && go mod download
 
-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
+# 复制完整源码并编译
+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