Update Dockerfile

Use proxy.golang.org to fetch dependencies faster/more reliably, produce
smaller upx-compressed binary.

Dockerfile

@@ -1,13 +1,14 @@
-FROM golang:latest as builder
-RUN CGO_ENABLED=0 GOOS=linux go install -a -installsuffix=nocgo std
-WORKDIR /project
-COPY go.sum go.mod ./
+FROM golang:alpine AS builder
+RUN apk add --update upx
+WORKDIR /app
+ENV GOPROXY=https://proxy.golang.org CGO_ENABLED=0
+COPY go.mod go.sum ./
 RUN go mod download
-COPY . .
-RUN go test ./...
-RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -installsuffix=nocgo -o=/tmp/bitmapist-server
+COPY . ./
+RUN go build -ldflags='-s -w' -o bitmapist-server && upx --lzma bitmapist-server
+
 FROM scratch
-COPY --from=builder /tmp/bitmapist-server .
+COPY --from=builder /app/bitmapist-server .
 VOLUME /data
 EXPOSE 6379
 CMD ["./bitmapist-server", "-addr=:6379", "-db=/data/bitmapist.db", "-bak=/data/bitmapist.bak"]