Cache attachments on external host with service worker (#7493)

app/javascript/mastodon/service_worker/entry.js

@@ -49,7 +49,7 @@ self.addEventListener('fetch', function(event) {
 
       return response;
     }));
-  } else if (storageFreeable && process.env.CDN_HOST ? url.host === process.env.CDN_HOST : url.pathname.startsWith('/system/')) {
+  } else if (storageFreeable && (ATTACHMENT_HOST ? url.host === ATTACHMENT_HOST : url.pathname.startsWith('/system/'))) {
     event.respondWith(openSystemCache().then(cache => {
       return cache.match(event.request.url).then(cached => {
         if (cached === undefined) {