nixos/docker: add daemonConfig option

Adds the virtualisation.docker.daemonConfig option that allows
changing Docker daemon settings as done in daemon.conf.

nixos/modules/virtualisation/docker.nix

@@ -8,7 +8,8 @@ let
 
   cfg = config.virtualisation.docker;
   proxy_env = config.networking.proxy.envVars;
-
+  daemonConfigJson = builtins.toJSON cfg.daemonConfig;
+  daemonConfigFile = pkgs.writeText "daemon.json" daemonConfigJson;
 in
 
 {
@@ -52,6 +53,20 @@ in
           '';
       };
 
+    daemonConfig =
+      mkOption {
+        type = types.anything;
+        default = { };
+        example = {
+          ipv6 = true;
+          "fixed-cidr-v6" = "fd00::/80";
+        };
+        description = ''
+          Configuration for docker daemon. The attributes are serialized to JSON used as daemon.conf.
+          See https://docs.docker.com/engine/reference/commandline/dockerd/#daemon-configuration-file
+        '';
+      };
+
     enableNvidia =
       mkOption {
         type = types.bool;
@@ -173,6 +188,7 @@ in
               ${cfg.package}/bin/dockerd \
                 --group=docker \
                 --host=fd:// \
+                --config-file=${daemonConfigFile} \
                 --log-driver=${cfg.logDriver} \
                 ${optionalString (cfg.storageDriver != null) "--storage-driver=${cfg.storageDriver}"} \
                 ${optionalString cfg.liveRestore "--live-restore" } \