Support -v flag to print version

If binary is called with -v, --v, -version, --version flags, or with a
single "version" argument, print line with program version to stdout and
exit with 0 code.

Closes #12

.github/workflows/docker.yml

@@ -30,7 +30,9 @@ jobs:
     steps:
     - uses: actions/checkout@v1
     - name: Build image
-      run: docker build . --tag image
+      run: |
+        echo ${GITHUB_REF##*/} | tee ci-version.txt
+        docker build . --tag image
     - name: Log into registry
       run: echo "${{ secrets.GITHUB_TOKEN }}" | docker login docker.pkg.github.com -u ${GITHUB_ACTOR} --password-stdin
     - name: Push image

Dockerfile

@@ -5,7 +5,11 @@ ENV GOPROXY=https://proxy.golang.org CGO_ENABLED=0
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . ./
-RUN go build -ldflags='-s -w' -o bitmapist-server && upx --lzma bitmapist-server
+RUN test -s ci-version.txt && \
+    go build -ldflags='-s -w' -o bitmapist-server \
+        -ldflags="-X=main.explicitVersion=$(cat ci-version.txt)" || \
+    go build -ldflags='-s -w' -o bitmapist-server \
+    && upx --lzma bitmapist-server
 
 FROM scratch
 COPY --from=builder /app/bitmapist-server .

main.go

@@ -3,11 +3,13 @@ package main
 
 import (
 	"flag"
+	"fmt"
 	"io/ioutil"
 	"log"
 	"os"
 	"os/signal"
 	"path/filepath"
+	"runtime/debug"
 	"syscall"
 	"time"
 
@@ -16,6 +18,8 @@ import (
 	"github.com/artyom/red"
 )
 
+var explicitVersion string // to be set by CI with -ldflags="-X=main.explicitVersion=v1.2.3"
+
 func main() {
 	args := struct {
 		Addr string `flag:"addr,address to listen"`
@@ -27,7 +31,21 @@ func main() {
 		File: "bitmapist.db",
 	}
 	autoflags.Define(&args)
+	var versionOnly bool
+	flag.BoolVar(&versionOnly, "v", versionOnly, "print version and exit")
+	flag.BoolVar(&versionOnly, "version", versionOnly, "print version and exit")
 	flag.Parse()
+	if versionOnly || (len(os.Args) == 2 && os.Args[1] == "version") {
+		v := "unknown"
+		if explicitVersion != "" {
+			v = explicitVersion
+		} else if bi, ok := debug.ReadBuildInfo(); ok {
+			v = bi.Main.Version
+		}
+		fmt.Printf("bitmapist-server %s\n", v)
+		return
+	}
+
 	log := log.New(os.Stderr, "", log.LstdFlags)
 
 	log.Println("loading data from", args.File)