Updates for Docker documentation. See discussion in #136. (#214)

Running-Mastodon/Production-guide.md

@@ -69,6 +69,7 @@ server {
 
   location ~ ^/(assets|system/media_attachments/files|system/accounts/avatars) {
     add_header Cache-Control "public, max-age=31536000, immutable";
+    try_files $uri @proxy;
   }
 
   location @proxy {