nixos/gitlab-runner: Add gitlab-runner.clear-docker-cache service

nixos/modules/services/continuous-integration/gitlab-runner.nix

@@ -453,6 +453,43 @@ in
         };
       });
     };
+    clear-docker-cache = {
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = lib.mdDoc ''
+          Whether to periodically prune gitlab runner's Docker resources. If
+          enabled, a systemd timer will run {command}`clear-docker-cache` as
+          specified by the `dates` option.
+        '';
+      };
+
+      flags = mkOption {
+        type = types.listOf types.str;
+        default = [ ];
+        example = [ "prune" ];
+        description = lib.mdDoc ''
+          Any additional flags passed to {command}`clear-docker-cache`.
+        '';
+      };
+
+      dates = mkOption {
+        default = "weekly";
+        type = types.str;
+        description = lib.mdDoc ''
+          Specification (in the format described by
+          {manpage}`systemd.time(7)`) of the time at
+          which the prune will occur.
+        '';
+      };
+
+      package = mkOption {
+        default = config.virtualisation.docker.package;
+        defaultText = literalExpression "config.virtualisation.docker.package";
+        example = literalExpression "pkgs.docker";
+        description = lib.mdDoc "Docker package to use for clearing up docker cache.";
+      };
+    };
   };
   config = mkIf cfg.enable {
     warnings = (mapAttrsToList
@@ -497,6 +534,22 @@ in
         KillMode = "process";
       };
     };
+    # Enable periodic clear-docker-cache script
+    systemd.services.gitlab-runner-clear-docker-cache = {
+      description = "Prune gitlab-runner docker resources";
+      restartIfChanged = false;
+      unitConfig.X-StopOnRemoval = false;
+
+      serviceConfig.Type = "oneshot";
+
+      path = [ cfg.clear-docker-cache.package pkgs.gawk ];
+
+      script = ''
+        ${pkgs.gitlab-runner}/bin/clear-docker-cache ${toString cfg.clear-docker-cache.flags}
+      '';
+
+      startAt = optional cfg.clear-docker-cache.enable cfg.clear-docker-cache.dates;
+    };
     # Enable docker if `docker` executor is used in any service
     virtualisation.docker.enable = mkIf (
       any (s: s.executor == "docker") (attrValues cfg.services)