nixos/docker: explicitly load kernel modules

This is analogous to #70447.

With security.lockKernelModules=true, docker commands result in the following
error without at least loading veth:

$ docker run hello-world
/nix/store/mr50kaan2vs4gc40ymwncb2vci25aq7z-docker-19.03.2/libexec/docker/docker: Error response from daemon: failed to create endpoint epic_kare on network bridge: failed to add the host (veth8b381f3) <=> sandbox (veth348e197) pair interfaces: operation not supported.
ERRO[0003] error waiting for container: context canceled

nixos/modules/virtualisation/docker.nix

@@ -149,6 +149,7 @@ in
   ###### implementation
 
   config = mkIf cfg.enable (mkMerge [{
+      boot.kernelModules = [ "bridge" "veth" ];
       environment.systemPackages = [ cfg.package ]
         ++ optional cfg.enableNvidia pkgs.nvidia-docker;
       users.groups.docker.gid = config.ids.gids.docker;