Heating up a container - most efficient
The problem with using the container as a resistor is that it's actually a very good conductor. Your wires will have more resistance, and will therefore get hotter than the container.

What would be amazing is if somehow you could deliver a very high current to the container, using a lower current flowing in the wires. Well, you can!

A very efficient way to heat an iron or steel object is by Induction Heating. It's used a lot in industry for heating bearings and things. It's also quite possible to do it yourself. Many people do.

It works by inducing an alternating current in the object to be heated. Essentially you're creating a transformer, where your object is the (single turn) secondary coil. Much more current can be made to flow in the secondary coil simply by using many turns in your primary.



An induction heater can get things really hot if you want:



And you can even use it to heat large containers: