Hey there!
'Container' was maybe not the best word to use, since it also includes campfires and fire pits. To cook, hold the ingredient you want to add, then /give it to the lightable object. Lightable objects which can hold ingredients are campfires, fire pits, and stoves. Once you've added all your ingredients, target the lightable object and type /cook. I've updated the /help description to hopefully be clearer. As it stands, corpses should be cookable. Let me know if that isn't working!
Additionally, the build I've just uploaded includes a form of butchering. Attacking an animal corpse with a bladed weapon will butcher it up into raw meat.
Thank you so much for your feedback, and thanks for playing!