feat: correctly handle shutdown signals

Add a new method to the application startup to listen for OS shutdown
signals and handle them appropriately. A shutdown signal will cause the
app to exit immediately.
Use correct syntax for the `ENTRYPOINT` field in the Dockerfile to ensure
that OS signals get passed down to the running application.

Dockerfile

@@ -20,4 +20,4 @@ WORKDIR /app
 # Copy compiled binary to release image
 COPY --from=build /build/src/exporter /app/fail2ban-prometheus-exporter
 
-ENTRYPOINT /app/fail2ban-prometheus-exporter
+ENTRYPOINT ["/app/fail2ban-prometheus-exporter"]

src/exporter.go

@@ -6,11 +6,13 @@ import (
 	"fail2ban-prometheus-exporter/collector/f2b"
 	"fail2ban-prometheus-exporter/collector/textfile"
 	"fmt"
-	"log"
-	"net/http"
-
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
+	"log"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
 )
 
 const (
@@ -54,6 +56,7 @@ func main() {
 	if appSettings.VersionMode {
 		printAppVersion()
 	} else {
+		handleGracefulShutdown()
 		log.Printf("fail2ban exporter version %s", version)
 		log.Printf("starting server at %s", appSettings.MetricsAddress)
 
@@ -85,3 +88,16 @@ func main() {
 		log.Print(err)
 	}
 }
+
+func handleGracefulShutdown() {
+	var signals = make(chan os.Signal)
+
+	signal.Notify(signals, syscall.SIGTERM)
+	signal.Notify(signals, syscall.SIGINT)
+
+	go func() {
+		sig := <-signals
+		log.Printf("caught signal: %+v", sig)
+		os.Exit(0)
+	}()
+}