diff --git a/common.nix b/common.nix
index 3cdb883..843b344 100644
--- a/common.nix
+++ b/common.nix
@@ -5,6 +5,24 @@
 
   time.timeZone = "Australia/Brisbane";
 
+  # Without this, the installer only ever sees cache.nixos.org, which
+  # doesn't carry sops-install-secrets (it's built straight from the
+  # sops-nix flake's own Go source, not part of nixpkgs) — every install
+  # would otherwise compile it from scratch, which is what ran an 8GB LXC
+  # container's disk out of space. Push a built copy to nix-cache once
+  # (from a machine with real disk headroom) and every future install,
+  # of any type, fetches instead of rebuilding.
+  nix.settings = {
+    substituters = [
+      "http://nix-cache"
+      "https://cache.nixos.org/"
+    ];
+    trusted-public-keys = [
+      "cache.local-1:usoWYanY3Kpq2+kDIS2nhWoLZiRxanmdysdzqCFBHW4="
+      "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
+    ];
+  };
+
   environment.systemPackages = with pkgs; [
     git
     curl