t490: add docker

Add Docker which is needed by DDEV, which I need to quickly get HTTP and
SSL running for a project.

There is https://devenv.sh/blog/2023/03/02/devenv-06-generating-containers-and-instant-shell-activation/#hosts-and-certificates
that mentions provisioning certificates for a local domain name, which
is something I'll look at in the future.

nix/hosts/t490/configuration.nix

@@ -8,6 +8,8 @@
     ./services
     ./users.nix
 
+    ./modules/docker.nix
+
     ../common
     ../../users/opdavies
   ];

nix/hosts/t490/modules/docker.nix

@@ -0,0 +1,5 @@
+{
+  virtualisation.docker.enable = true;
+
+  users.users.opdavies.extraGroups = [ "docker" ];
+}