docker: Minor improvements, fix failing test

- Replace usage of deprecated CLI flag `--daemon`
- Introduce `storageDriver` option for module
- Fix failing test by using `overlay` storage driver

nixos/modules/virtualisation/docker.nix

@@ -43,6 +43,17 @@ in
             in future.  So set this option explicitly to false if you wish.
           '';
       };
+    storageDriver =
+      mkOption {
+        type = types.enum ["aufs" "btrfs" "devicemapper" "overlay" "zfs"];
+        description =
+          ''
+            This option determines which Docker storage driver to use.
+            It is required but lacks a default value as its most
+            suitable value will depend the filesystems available on the
+            host.
+          '';
+      };
     extraOptions =
       mkOption {
         type = types.separatedString " ";
@@ -85,7 +96,7 @@ in
         after = [ "network.target" "docker.socket" ];
         requires = [ "docker.socket" ];
         serviceConfig = {
-          ExecStart = "${pkgs.docker}/bin/docker --daemon=true --host=fd:// --group=docker ${cfg.extraOptions}";
+          ExecStart = "${pkgs.docker}/bin/docker daemon --host=fd:// --group=docker --storage-driver=${cfg.storageDriver} ${cfg.extraOptions}";
           #  I'm not sure if that limits aren't too high, but it's what
           #  goes in config bundled with docker itself
           LimitNOFILE = 1048576;
@@ -111,7 +122,7 @@ in
         wantedBy = [ "multi-user.target" ];
         after = [ "network.target" ];
         serviceConfig = {
-          ExecStart = "${pkgs.docker}/bin/docker --daemon=true --group=docker ${cfg.extraOptions}";
+          ExecStart = "${pkgs.docker}/bin/docker daemon --group=docker --storage-driver=${cfg.storageDriver} ${cfg.extraOptions}";
           #  I'm not sure if that limits aren't too high, but it's what
           #  goes in config bundled with docker itself
           LimitNOFILE = 1048576;

nixos/tests/docker.nix

@@ -11,6 +11,7 @@ import ./make-test.nix ({ pkgs, ...} : {
       { config, pkgs, ... }:
         {
           virtualisation.docker.enable = true;
+          virtualisation.docker.storageDriver = "overlay";
         };
     };