support loading custom frontend templates from file

.gitignore

@@ -2,5 +2,6 @@
 /docker/docker-compose.*.yml
 /ddns
 dump.rdb
+index.html
 /docker/.caddy_mount/
 /docker/.redis_mount/

frontend/frontend.go

@@ -41,7 +41,7 @@ func (f *Frontend) Run() error {
 		r.Use(gin.Logger())
 	}
 
-	r.SetHTMLTemplate(buildTemplate())
+	r.SetHTMLTemplate(buildTemplate(f.config.CustomTemplatePath))
 
 	r.GET("/", func(g *gin.Context) {
 		g.HTML(200, "index.html", gin.H{"domain": f.config.Domain})
@@ -153,10 +153,18 @@ func extractRemoteAddr(req *http.Request) (string, error) {
 }
 
 // Get index template from bindata
-func buildTemplate() *template.Template {
+func buildTemplate(customTemplatePath string) *template.Template {
-	html, err := template.New("index.html").Parse(indexTemplate)
+	var html *template.Template
+	var err error
+
+	if customTemplatePath != "" {
+		html, err = template.ParseFiles(customTemplatePath)
+	} else {
+		html, err = template.New("index.html").Parse(indexTemplate)
+	}
+
 	if err != nil {
-		log.Fatal(err)
+		log.Fatalf("Error parsing frontend template: %v", err)
 	}
 
 	return html

shared/config.go

@@ -15,6 +15,7 @@ type Config struct {
 	ListenFrontend     string
 	ListenBackend      string
 	RedisHost          string
+	CustomTemplatePath string
 
 	fs *flag.FlagSet
 }
@@ -33,6 +34,9 @@ func (c *Config) Initialize() {
 	fs.StringVar(&c.ListenFrontend, "listen-frontend", ":8080",
 		"Which socket should the frontend web service use to bind itself")
 
+	fs.StringVar(&c.CustomTemplatePath, "custom-template-path", "",
+		"A path to a custom `index.html` file that will be used instead of the default frontend template")
+
 	fs.StringVar(&c.RedisHost, "redis-host", ":6379",
 		"The Redis socket that should be used")
 
@@ -46,7 +50,9 @@ func (c *Config) Initialize() {
 }
 
 func (c *Config) Validate() {
-	c.fs.Parse(os.Args[1:])
+	if err := c.fs.Parse(os.Args[1:]); err != nil {
+		log.Fatalf("Error parsing configuration: %v", err)
+	}
 
 	if c.Domain == "" {
 		log.Fatal("You have to supply the domain via env variable DDNS_DOMAIN or command line flag --domain=DOMAIN")