Add Cache-Control for serving static files

Static files will be rechecked every 3 days instead of unlimited cache time. The Cache-Control header is mandatory in order to make browsers handle cache properly with Go's FileServer. The option can be modified in the server.CacheControl line of the config file.

pkg/config/config.yaml

@@ -59,6 +59,7 @@ coordinator:
   # HTTP(S) server config
   server:
     address: :8000
+    cacheControl: "max-age=259200, must-revalidate"
     https: false
     # Letsencrypt or self cert config
     tls:

pkg/config/shared.go

@@ -30,9 +30,10 @@ type Monitoring struct {
 func (c *Monitoring) IsEnabled() bool { return c.MetricEnabled || c.ProfilingEnabled }
 
 type Server struct {
-	Address string
-	Https   bool
-	Tls     struct {
+	Address      string
+	CacheControl string
+	Https        bool
+	Tls          struct {
 		Address   string
 		Domain    string
 		HttpsKey  string

pkg/coordinator/coordinator.go

@@ -92,6 +92,9 @@ func index(conf config.CoordinatorConfig, log *logger.Logger) httpx.Handler {
 	if conf.Coordinator.Debug {
 		log.Info().Msgf("Using auto-reloading index.html")
 		return httpx.HandlerFunc(func(w httpx.ResponseWriter, r *httpx.Request) {
+			if conf.Coordinator.Server.CacheControl != "" {
+				w.Header().Add("Cache-Control", conf.Coordinator.Server.CacheControl)
+			}
 			if r.URL.Path == "/" || strings.HasSuffix(r.URL.Path, "/index.html") {
 				tpl := template.Must(template.ParseFiles(indexHTML))
 				handler(tpl, w, r)
@@ -102,6 +105,9 @@ func index(conf config.CoordinatorConfig, log *logger.Logger) httpx.Handler {
 	}
 
 	return httpx.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if conf.Coordinator.Server.CacheControl != "" {
+			w.Header().Add("Cache-Control", conf.Coordinator.Server.CacheControl)
+		}
 		if r.URL.Path == "/" || strings.HasSuffix(r.URL.Path, "/index.html") {
 			handler(indexTpl, w, r)
 			return