fix: simplify Dockerfile build process to avoid missing go.sum issue

Dockerfile

@@ -3,13 +3,13 @@ FROM golang:1.21-alpine AS build
 
 WORKDIR /build
 
-# 复制源码
-COPY go.mod go.sum ./
-RUN apk add --no-cache git && go mod download
-
-# 复制完整源码并编译
+# 直接复制所有源码
 COPY . .
-RUN CGO_ENABLED=0 GOOS=linux go build -trimpath -ldflags="-s -w" -o /app/alist main.go
+
+# 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
 
 # 生产阶段
 FROM alpine:3.19