nixos: Don't enable Docker by default

Regression introduced by c94005358c.

The commit introduced declarative docker containers and subsequently
enables docker whenever any declarative docker containers are defined.

This is done via an option with type "attrsOf somesubmodule" and a check
on whether the attribute set is empty.

Unfortunately, the check was whether a *list* is empty rather than
wether an attribute set is empty, so "mkIf (cfg != [])" *always*
evaluates to true and thus subsequently enables docker by default:

$ nix-instantiate --eval nixos --arg configuration {} \
    -A config.virtualisation.docker.enable
true

Fixing this is simply done by changing the check to "mkIf (cfg != {})".

Tested this by running the "docker-containers" NixOS test and it still
passes.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @benley, @danbst, @Infinisil, @nlewo

nixos/modules/virtualisation/docker-containers.nix

@@ -222,7 +222,7 @@ in {
     description = "Docker containers to run as systemd services.";
   };
 
-  config = mkIf (cfg != []) {
+  config = mkIf (cfg != {}) {
 
     systemd.services = mapAttrs' (n: v: nameValuePair "docker-${n}" (mkService n v)) cfg;