feat(docker): faster multi-platform builds

See: https://www.docker.com/blog/faster-multi-platform-builds-dockerfile-cross-compilation-guide/

Dockerfile

@@ -1,20 +1,22 @@
 # build stage
-FROM golang:1.19 AS builder
+FROM --platform=$BUILDPLATFORM golang:1.19-alpine AS builder
 
 WORKDIR /app
 COPY . .
-RUN go env -w GO111MODULE=on \
+ARG TARGETOS TARGETARCH
+RUN sed -i 's/dl-cdn.alpinelinux.org/mirrors.tuna.tsinghua.edu.cn/g' /etc/apk/repositories \
+    && apk add git make tzdata \
+    && go env -w GO111MODULE=on \
     && go env -w GOPROXY=https://goproxy.cn,direct \
-    && make clean build
+    && GOOS=$TARGETOS GOARCH=$TARGETARCH make clean build
 
 # final stage
 FROM alpine
 LABEL name=ddns-go
 LABEL url=https://github.com/jeessy2/ddns-go
 
 WORKDIR /app
-RUN sed -i 's/dl-cdn.alpinelinux.org/mirrors.tuna.tsinghua.edu.cn/g' /etc/apk/repositories \
-    && apk add --no-cache tzdata
+COPY --from=builder /usr/share/zoneinfo /usr/share/zoneinfo
 ENV TZ=Asia/Shanghai
 COPY --from=builder /app/ddns-go /app/ddns-go
 EXPOSE 9876