Use golang's cross-compilation capabilities

Dockerfile

@@ -1,10 +1,13 @@
-FROM golang:1.21-alpine AS builder
+FROM --platform=$BUILDPLATFORM golang:1.21-alpine AS builder
 LABEL maintainer="nekohasekai <[email protected]>"
 COPY . /go/src/github.com/sagernet/sing-box
 WORKDIR /go/src/github.com/sagernet/sing-box
+ARG TARGETOS TARGETARCH
 ARG GOPROXY=""
 ENV GOPROXY ${GOPROXY}
 ENV CGO_ENABLED=0
+ENV GOOS=$TARGETOS
+ENV GOARCH=$TARGETARCH
 RUN set -ex \
     && apk add git build-base \
     && export COMMIT=$(git rev-parse --short HEAD) \
@@ -13,11 +16,11 @@ RUN set -ex \
         -o /go/bin/sing-box \
         -ldflags "-X \"github.com/sagernet/sing-box/constant.Version=$VERSION\" -s -w -buildid=" \
         ./cmd/sing-box
-FROM alpine AS dist
+FROM --platform=$TARGETPLATFORM alpine AS dist
 LABEL maintainer="nekohasekai <[email protected]>"
 RUN set -ex \
     && apk upgrade \
     && apk add bash tzdata ca-certificates \
     && rm -rf /var/cache/apk/*
 COPY --from=builder /go/bin/sing-box /usr/local/bin/sing-box
-ENTRYPOINT ["sing-box"]
+ENTRYPOINT ["sing-box"]