feat: 添加后端 Dockerfile

- 多阶段构建：golang:1.24-alpine 编译，alpine:3.20 运行
- CGO_ENABLED=0 静态链接，包含 ca-certificates 和时区数据

backend/Dockerfile

@@ -0,0 +1,27 @@
+# ---- 构建阶段 ----
+FROM golang:1.26-alpine AS builder
+
+WORKDIR /app
+
+# 先复制依赖清单，利用 Docker 缓存层
+COPY go.mod go.sum ./
+RUN go mod download
+
+# 复制源码并构建
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -o /camtalk ./cmd/server
+
+# ---- 运行阶段 ----
+FROM alpine:3.20
+
+RUN apk add --no-cache ca-certificates tzdata
+
+WORKDIR /app
+
+# 复制二进制和配置
+COPY --from=builder /camtalk .
+COPY config.yaml .
+
+EXPOSE 8080
+
+ENTRYPOINT ["./camtalk"]