Evaluate skycam upstream at runtime

modules/services/nginx/default.nix

@@ -118,8 +118,10 @@ in {
           serverAliases = [ "www.jdholt.com" ];
           extraConfig = nginxErrorPages + nginxEdgeHeaders + nginxStrictHeaders;
           locations."/skycam/snapshot.jpg" = {
-            proxyPass = "http://skycam.mesh.vimium.net:8080/snapshot";
             extraConfig = ''
+              set $backend "skycam.mesh.vimium.net:8080";
+
+              proxy_pass http://$backend/snapshot;
               proxy_cache skycam_cache;
               proxy_cache_valid any 10s;
               proxy_ignore_headers Cache-Control Expires Set-Cookie;