fix: weekly docker prune

modules/docker.nix

@@ -13,20 +13,38 @@
       # As we use btrfs, we enable the according storageDriver option.
       storageDriver = "btrfs";
 
-      # Run `docker system prune -f` every week.
+      # Run `docker system prune -f ${autoPrune.flags}` every week.
       autoPrune.enable = true;
       autoPrune.dates = "weekly";
       # https://docs.docker.com/engine/reference/commandline/system_prune/#options
       autoPrune.flags = [
         "--all"
-        "--volumes"
         # https://docs.docker.com/engine/reference/commandline/system_prune/#filter
         # https://pkg.go.dev/maze.io/x/duration#ParseDuration
-        "--filter until=7d"
+        # `7d` could not be parsed, thus we use `168h`
+        "--filter until=168h"
+        # `--volumes` can't be used together with `--filter`.
+        # Thus we added our own service below.
+        #--volumes
       ];
     };
   };
 
+  # Prune docker volumes.
+  # This is a slightly modified copy of https://github.com/NixOS/nixpkgs/blob/5a237aecb57296f67276ac9ab296a41c23981f56/nixos/modules/virtualisation/docker.nix#L211C5-L226
+  systemd.services."docker-prune-volumes" = {
+    description = "Prune docker volumes";
+    restartIfChanged = false;
+    unitConfig.X-StopOnRemoval = false;
+    serviceConfig.Type = "oneshot";
+    script = ''
+      ${pkgs.docker}/bin/docker system prune -f --all --volumes
+    '';
+    startAt = "weekly";
+    after = [ "docker.service" ];
+    requires = [ "docker.service" ];
+  };
+
   # Monitor unhealthy Docker containers.
   systemd.timers."docker-health" = {
     wantedBy = [ "timers.target" ];