fix: 锁定基础镜像版本，解决 Docker 镜像拉取超时问题

backend/gateway/Dockerfile

@@ -8,7 +8,7 @@ COPY main.go ./
 RUN go mod download
 RUN go build -o gateway main.go
 
-FROM alpine:latest
+FROM alpine:3.23
 
 WORKDIR /app
 

backend/services/user-svc/Dockerfile

@@ -12,7 +12,7 @@ RUN go install google.golang.org/grpc/cmd/protoc-gen-go-grpc@latest
 RUN protoc --go_out=. --go-grpc_out=. user.proto
 RUN go build -o user-svc main.go
 
-FROM alpine:latest
+FROM alpine:3.23
 
 WORKDIR /app