fix: resolve container backends dynamically to allow nginx startup

conf.d/openlist.conf.template

@@ -1,3 +1,8 @@
+upstream openlist_backend {
+    zone openlist_backend 64k;
+    server openlist:5244 resolve;
+}
+
 server {
     include snippets/https.conf;
 
@@ -6,7 +11,7 @@ server {
     include snippets/proxy.conf;
 
     location / {
-        proxy_pass http://openlist:5244;
+        proxy_pass http://openlist_backend;
         proxy_redirect off;
     }
 }