add: apple container build target

pkgs/default.nix

@@ -4,7 +4,10 @@
   caitsith ? null
 }:
 {
+  # any overlays we define here
   atuinLatest = pkgs.atuinLatest;
+  containerLatest = pkgs.containerLatest;
+
   # any packages that are added to pkgs and are inputs, we inherit them here
   inherit hildibrand;
   inherit caitsith;