Pass substituters in so local builds aren't trying to hit the in-kubernetes cache.

docker/server/Dockerfile

@@ -2,15 +2,15 @@
 #
 # Builder
 #
+ARG NIX_SUBSTITUTERS=https://cache.nixos.org
 
 FROM nixos/nix:2.31.3 AS builder
 
+ARG NIX_SUBSTITUTERS
 RUN tee -a /etc/nix/nix.conf <<EOF
 extra-experimental-features = nix-command flakes
 filter-syscalls = false
-substituters = http://ncps.nix-pull-through-cache.svc.cluster.local:80 https://cache.nixos.org
-download-attempts = 1
-connect-timeout = 3
+substituters = $NIX_SUBSTITUTERS
 EOF
 
 RUN cp "$(nix build nixpkgs#cacert --print-out-paths)/etc/ssl/certs/ca-bundle.crt" /tmp/ca-bundle.crt