Setup an Hydra declarative project

This commit adds configuration files ingested an Hydra instance to
create a declarative project.
Currently, this Hydra instance builds
- the master branch
- all pull requests

We use the nix community provided Hydra instance
https://hydra.nix-community.org/project/simple-nixos-mailserver.

.hydra/declarative-jobsets.nix

@@ -0,0 +1,58 @@
+{ nixpkgs, declInput, pulls }:
+
+let
+  pkgs = import nixpkgs {};
+
+  prs = builtins.fromJSON (builtins.readFile pulls);
+  prJobsets =  pkgs.lib.mapAttrs (num: info:
+    { enabled = 1;
+      hidden = false;
+      description = "PR ${num}: ${info.title}";
+      nixexprinput = "snm";
+      nixexprpath = ".hydra/default.nix";
+      checkinterval = 30;
+      schedulingshares = 20;
+      enableemail = false;
+      emailoverride = "";
+      keepnr = 1;
+      type = 0;
+      inputs = {
+        cloudwatt = {
+          type = "git";
+          value = "${info.target_repo_url} merge-requests/${info.iid}/head";
+          emailresponsible = false;
+        };
+      };
+    }
+  ) prs;
+
+  desc = prJobsets // {
+    master = {
+      description = "Build master branch of Simple NixOS MailServer";
+      checkinterval = "60";
+      enabled = "1";
+      nixexprinput = "snm";
+      nixexprpath = ".hydra/default.nix";
+      schedulingshares = 100;
+      enableemail = false;
+      emailoverride = "";
+      keepnr = 3;
+      hidden = false;
+      type = 0;
+      inputs = {
+        snm = {
+          value = "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver master";
+          type = "git";
+          emailresponsible = false;
+        };
+      };
+    };
+  };
+
+in {
+  jobsets = pkgs.runCommand "spec-jobsets.json" {} ''
+    cat >$out <<EOF
+    ${builtins.toJSON desc}
+    EOF
+  '';
+}

.hydra/default.nix

@@ -0,0 +1 @@
+import ../tests

.hydra/spec.json

@@ -0,0 +1,30 @@
+{
+  "enabled": 1,
+  "hidden": false,
+  "description": "Simple NixOS Mailserver",
+  "nixexprinput": "nixexpr",
+  "nixexprpath": ".hydra/declarative-jobsets.nix",
+  "checkinterval": 60,
+  "schedulingshares": 100,
+  "enableemail": false,
+  "emailoverride": "",
+  "keepnr": 3,
+  "type": 0,
+  "inputs": {
+    "nixexpr": {
+      "value": "https://gitlab.com/simple-nixos-mailserver/nixos-mailserver master",
+      "type": "git",
+      "emailresponsible": false
+    },
+    "nixpkgs": {
+      "value": "https://github.com/NixOS/nixpkgs 0f920b05cbcdb8c0f3c5c4a8ea29f1f0065c7033 ",
+      "type": "git",
+      "emailresponsible": false
+    },
+    "pulls": {
+      "type": "gitlabpulls",
+      "value": "https://gitlab.com 7219050",
+      "emailresponsible": false
+    }
+  }
+}