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/solr.nix

@@ -0,0 +1,43 @@
+{ config, pkgs, lib, ... }:
+
+# This module sets up solr with one core. It is a bit tedious…. If you
+# know a better solution, please let me know.
+{
+  nixpkgs.config = {
+    permittedInsecurePackages = [
+      "solr-8.6.3"
+      # NOTE: Qtwebkit is a dep for wkhtmltopdf, this line is needed until #201765 is fixed in nixpkgs
+      "qtwebkit-5.212.0-alpha4"
+    ];
+  };
+
+  services.solr = {
+    enable = true;
+  };
+  # This is needed to run solr script as user solr
+  users.users.solr.useDefaultShell = true;
+
+  systemd.services.solr-init =
+    let
+      solrPort = toString config.services.solr.port;
+      initSolr = ''
+        if [ ! -f ${config.services.solr.stateDir}/docspell_core ]; then
+          while ! echo "" | ${pkgs.inetutils}/bin/telnet localhost ${solrPort}
+          do
+             echo "Waiting for SOLR become ready..."
+             sleep 1.5
+          done
+          ${pkgs.su}/bin/su -s ${pkgs.bash}/bin/sh solr -c "${pkgs.solr}/bin/solr create_core -c docspell -p ${solrPort}";
+          touch ${config.services.solr.stateDir}/docspell_core
+        fi
+      '';
+    in
+    {
+      script = initSolr;
+      after = [ "solr.target" ];
+      wantedBy = [ "multi-user.target" ];
+      requires = [ "solr.target" ];
+      description = "Create a core at solr";
+    };
+
+}