fix: resolve container backends dynamically to allow nginx startup

conf.d/gitea.conf.template

@@ -1,3 +1,8 @@
+upstream gitea_backend {
+    zone gitea_backend 64k;
+    server gitea:3000 resolve;
+}
+
 server {
     include snippets/https.conf;
 
@@ -6,7 +11,7 @@ server {
     include snippets/proxy.conf;
 
     location / {
-        proxy_pass http://gitea:3000;
+        proxy_pass http://gitea_backend;
         client_max_body_size 512M;
     }
 }