Installer: Add Cache-Control header to static assets

This fixes UX where Launcher iframe blanks when navigating in the app

Change-Id: I2966ef383c77d2986b6892f3828581b7785e90b0
diff --git a/core/installer/welcome/env.go b/core/installer/welcome/env.go
index 15be0eb..5d2206f 100644
--- a/core/installer/welcome/env.go
+++ b/core/installer/welcome/env.go
@@ -125,7 +125,7 @@
 
 func (s *EnvServer) Start() {
 	r := mux.NewRouter()
-	r.PathPrefix("/static/").Handler(http.FileServer(http.FS(staticAssets)))
+	r.PathPrefix("/static/").Handler(cachingHandler{http.FileServer(http.FS(staticAssets))})
 	r.Path("/env/{key}").Methods("GET").HandlerFunc(s.monitorTask)
 	r.Path("/env/{key}").Methods("POST").HandlerFunc(s.publishDNSRecords)
 	r.Path("/").Methods("GET").HandlerFunc(s.createEnvForm)