nix: implement flake

This commit introduces the ability to install Docspell from a Nix flake.
There are no major changes to the logic of the previous modules outside of
organizing them in a flake and adding a simple check script.

nix/checks/configuration-test.nix

@@ -0,0 +1,83 @@
+{ config, pkgs, ... }:
+let
+  full-text-search = {
+    enabled = true;
+    solr.url = "http://localhost:${toString config.services.solr.port}/solr/docspell";
+    postgresql = {
+      pg-config = {
+        "german" = "my-germam";
+      };
+    };
+  };
+in
+{
+
+  i18n = {
+    defaultLocale = "en_US.UTF-8";
+  };
+  console.keyMap = "de";
+
+  users.users.root = {
+    password = "root";
+  };
+
+
+  services.docspell-joex = {
+    enable = true;
+    waitForTarget = "solr-init.target";
+    bind.address = "0.0.0.0";
+    base-url = "http://localhost:7878";
+    jvmArgs = [ "-J-Xmx1536M" ];
+    inherit full-text-search;
+  };
+  services.docspell-restserver = {
+    enable = true;
+    bind.address = "0.0.0.0";
+    backend = {
+      addons.enabled = true;
+    };
+    integration-endpoint = {
+      enabled = true;
+      http-header = {
+        enabled = true;
+        header-value = "test123";
+      };
+    };
+    openid = [
+      {
+        enabled = true;
+        display = "Local";
+        provider = {
+          provider-id = "local";
+          client-id = "cid1";
+          client-secret = "csecret-1";
+          authorize-url = "http://auth";
+          token-url = "http://token";
+          sign-key = "b64:uiaeuae";
+        };
+      }
+    ];
+    inherit full-text-search;
+  };
+
+  environment.systemPackages =
+    [
+      pkgs.jq
+      pkgs.inetutils
+      pkgs.htop
+      pkgs.openjdk
+    ];
+
+
+  services.xserver = {
+    enable = false;
+  };
+
+  networking = {
+    hostName = "docspelltest";
+    firewall.allowedTCPPorts = [ 7880 ];
+  };
+
+  system.stateVersion = "22.05";
+
+}