Cache nix store between builds

.drone.jsonnet

@@ -1,8 +1,45 @@
+local Cache(name, settings) = {
+  name: name,
+  image: "meltwater/drone-cache",
+  pull: true,
+  settings: {
+    backend: "filesystem",
+    cache_key: "{{ .Repo.Name }}",
+    archive_format: "gzip",
+    mount: [
+      "/nix/store"
+    ]
+  } + settings,
+  volumes: [
+    {
+      name: "cache",
+      path: "/tmp/cache"
+    }
+  ]
+};
+
 local Pipeline(channel) = {
   kind: "pipeline",
   type: "docker",
   name: channel,
+  // This is ignored by the drone-cli YAML translator, so unfortunately, it has
+  // to be copy-pasted into relevant steps.
+  environment: {
+    NIXPKGS_ALLOW_UNFREE: 1,
+    NUR_REPO: "xeals",
+    CACHIX_CACHE: "xeals",
+    CACHIX_SIGNING_KEY: { from_secret: "cachix_key" },
+  },
+  volumes: [
+    {
+      name: "cache",
+      temp: {}
+    }
+  ],
   steps: [
+    Cache("restore-nix-store", {
+      restore: true
+    }),
     {
       name: "build",
       image: "nixos/nix",
@@ -32,7 +69,12 @@ local Pipeline(channel) = {
         CACHIX_CACHE: "xeals",
         CACHIX_SIGNING_KEY: { from_secret: "cachix_key" },
       },
-    }
+    },
+    Cache("save-nix-store", {
+      rebuild: true
+    }) + {
+      when: { status: [ "success", "failure" ] }
+    },
   ]
 };