create module for disk configuration

modules/custom/default.nix

@@ -1,3 +1,3 @@
 {
-  imports = [ ./terraform-database.nix ./data-disk.nix ./ssh-certificates.nix ./k3s ];
+  imports = [ ./terraform-database.nix ./data-disk.nix ./ssh-certificates.nix ./k3s ./disko.nix ];
 }

modules/custom/disko.nix

@@ -0,0 +1,43 @@
+{ lib, config, ... }:
+let cfg = config.custom.disko;
+in {
+  options = {
+    custom = {
+      disko.osDiskDevice = lib.mkOption {
+        type = lib.types.str;
+        description = ''
+          The disk device to be used for the operating system.
+        '';
+      };
+    };
+  };
+
+  # TODO: rename this to 'osDisk'. Unfortunately, we would need to run nixos-anywhere again then
+  config.disko.devices.disk.vdb = {
+    device = cfg.osDiskDevice;
+    type = "disk";
+    content = {
+      type = "gpt";
+      partitions = {
+        ESP = {
+          type = "EF00";
+          size = "500M";
+          content = {
+            type = "filesystem";
+            format = "vfat";
+            mountpoint = "/boot";
+          };
+        };
+        root = {
+          end = "-4G";
+          content = {
+            type = "filesystem";
+            format = "btrfs";
+            mountpoint = "/";
+          };
+        };
+        swap = { size = "100%"; };
+      };
+    };
+  };
+}