nixos: Don't enable Docker by default

Regression introduced by c9400535

.

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