feat(ops/modules): Add module for running Nixery

This sets up a very simple Nixery instance with some things lacking:

* no support for garbage-collecting image fragments (yet)
* no popularity setup

The plan is to use this to get the ball rolling on a separate
domain (e.g. images.tvl.fyi), iron things out and then look into
flipping over nixery.dev

Change-Id: Ic594809f9d487fec7a0f632d608752a3f9c61315
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3280
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Reviewed-by: sterni <sternenseemann@systemli.org>

.nixery/default.nix

@@ -1,5 +1,5 @@
 # See README.md
-{ depot ? import ../. {} }:
+{ depot ? import ../. {}, ... }:
 
 depot.third_party.nixpkgs.extend(_: _: {
   tvl = depot;