Fact is, building it "properly" with concrete will increase the value of the house, building it with a shipping container will likely reduce it.
ultimately people will think(even if not true) that a shipping container in the ground is stupid, going to rust and need dealing with at some stage, and they are probably right. So when you sell, if you sell, people will be thinking they like the cave, but one day its going to cost thousands to remove, or many more thousands to replace and get done right. If you build a proper concrete viable for the life of the house with no issues, it will cost more but will be a selling point for the house, thus, they money isn't being wasted so to speak.
Obviously if a done to the highest standards of construction would cost £30k and a shipping container one would cost £10k and thats all you have and no chance of having that kind of money soon going for a cheaper option is more viable, but long term I think a more "proper" build that other people buying a house would be comfortable with makes more financial sense.