add parameterization for host and port

Host and port can be specified by either a environment variable or a
dedicated command line parameter. Command line parameters will be preferred
over environment variables, which will be preferred over the application
defaults.

main.go

@@ -3,9 +3,12 @@ package main
 import (
 	"bytes"
 	"compress/gzip"
+	"flag"
+	"fmt"
 	"log"
 	"net/http"
 	"os"
+	"strconv"
 	"strings"
 	"time"
 
@@ -21,9 +24,41 @@ var (
 	cachedServiceResults          []byte
 	cachedServiceResultsGzipped   []byte
 	cachedServiceResultsTimestamp time.Time
+	port                          int
+	host                          string
 )
 
+func init() {
+	// Customizing priority:
+	// (1) command line parameters will be preferred over
+	// (2) environment variables will be preferred over
+	// (3) application defaults
+
+	// set defaults for the case that neither an environment variable nor a
+	// command line parameter is passed
+	var defaultHost = ""
+	var defaultPort = 8080
+
+	// assume set if the is a valid port number
+	if p, err := strconv.Atoi(os.Getenv("GATUS_CONFIG_PORT")); err == nil && p > 0 {
+		defaultPort = p
+	}
+
+	// explicitly asked if the user has set a the environment variable to
+	// blank / empty in order to allow listening on all interfaces
+	if h, set := os.LookupEnv("GATUS_CONFIG_HOST"); set == true {
+		defaultHost = h
+	}
+
+	flag.IntVar(&port, "port", defaultPort, "port to listen (default: 8080)")
+	flag.IntVar(&port, "p", defaultPort, "port to listen (default: 8080 ; shorthand)")
+	flag.StringVar(&host, "host", defaultHost, "host to listen on (default all interfaces on host)")
+	flag.StringVar(&host, "host", defaultHost, "host to listen on (default all interfaces on host; shorthand)")
+}
+
 func main() {
+	flag.Parse()
+
 	cfg := loadConfiguration()
 	resultsHandler := serviceResultsHandler
 	if cfg.Security != nil && cfg.Security.IsValid() {
@@ -35,9 +70,10 @@ func main() {
 	if cfg.Metrics {
 		http.Handle("/metrics", promhttp.Handler())
 	}
-	log.Println("[main][main] Listening on port 8080")
+
+	log.Printf("[main][main] Listening on %s:%d", host, port)
 	go watchdog.Monitor(cfg)
-	log.Fatal(http.ListenAndServe(":8080", nil))
+	log.Fatal(http.ListenAndServe(fmt.Sprintf("%s:%d", host, port), nil))
 }
 
 func loadConfiguration() *config.Config {