ecs-agent NixOS module: init

A very simple skeleton for now that doesn't attempt to model any of
the agent configuration, but we can grow it later. Tested and works
on an EC2 instance with ECS.

nixos/modules/module-list.nix

@@ -645,6 +645,7 @@
   ./virtualisation/container-config.nix
   ./virtualisation/containers.nix
   ./virtualisation/docker.nix
+  ./virtualisation/ecs-agent.nix
   ./virtualisation/libvirtd.nix
   ./virtualisation/lxc.nix
   ./virtualisation/lxcfs.nix

nixos/modules/virtualisation/ecs-agent.nix

@@ -0,0 +1,42 @@
+{ config, pkgs, lib, ... }:
+
+with lib;
+
+let
+  cfg = config.services.ecs-agent;
+in {
+  options.services.ecs-agent = {
+    enable = mkEnableOption "Amazon ECS agent";
+
+    package = mkOption {
+      type = types.path;
+      description = "The ECS agent package to use";
+      default = pkgs.ecs-agent;
+    };
+
+    extra-environment = mkOption {
+      type = types.attrsOf types.str;
+      description = "The environment the ECS agent should run with. See the ECS agent documentation for keys that work here.";
+      default = {};
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    systemd.services.ecs-agent = {
+      inherit (cfg.package.meta) description;
+      after    = [ "network.target" ];
+      wantedBy = [ "multi-user.target" ];
+
+      environment = cfg.extra-environment;
+
+      script = ''
+        if [ ! -z "$ECS_DATADIR" ]; then
+          echo "FOOOO"
+          mkdir -p "$ECS_DATADIR"
+        fi
+        ${cfg.package.bin}/bin/agent
+      '';
+    };
+  };
+}
+