fix: resolve container backends dynamically to allow nginx startup

conf.d/immich.conf.template

@@ -1,3 +1,8 @@
+upstream immich_backend {
+    zone immich_backend 64k;
+    server immich-server:2283 resolve;
+}
+
 server {
     include snippets/https.conf;
 
@@ -20,7 +25,7 @@ server {
     send_timeout       600s;
 
     location / {
-        proxy_pass http://immich-server:2283;
+        proxy_pass http://immich_backend;
         proxy_redirect off;
     }