nix: add development vm

This commit adds back the ability to run a development VM with docspell.
See updated documentation for examples.

nix/dev-vm/default.nix

@@ -0,0 +1,28 @@
+# NOTE: modulesPath and imports are taken from nixpkgs#59219
+{ modulesPath, pkgs, lib, ... }: {
+  imports = [ (modulesPath + "/virtualisation/qemu-vm.nix") ];
+  services.openssh = {
+    enable = true;
+    permitRootLogin = "yes";
+  };
+
+  services.docspell-restserver = {
+    openid = lib.mkForce [ ];
+    backend = lib.mkForce {
+      signup = {
+        mode = "open";
+      };
+    };
+  };
+
+  # Otherwise oomkiller kills docspell
+  virtualisation.memorySize = 2048;
+
+  virtualisation.forwardPorts = [
+    # SSH
+    { from = "host"; host.port = 64022; guest.port = 22; }
+    # Docspell
+    { from = "host"; host.port = 64080; guest.port = 7880; }
+  ];
+
+}