add pack and round count to containers

lib/cannery/containers.ex

@@ -207,6 +207,18 @@ defmodule Cannery.Containers do
     if count == 0, do: raise("could not delete container tag"), else: count
   end
 
+  @doc """
+  Returns number of rounds in container. If data is already preloaded, then
+  there will be no db hit.
+  """
+  @spec get_container_ammo_group_count!(Container.t()) :: non_neg_integer()
+  def get_container_ammo_group_count!(%Container{} = container) do
+    container
+    |> Repo.preload(:ammo_groups)
+    |> Map.fetch!(:ammo_groups)
+    |> Enum.count()
+  end
+
   @doc """
   Returns number of rounds in container. If data is already preloaded, then
   there will be no db hit.