Brief scenario: Evil Genius (EG) with a lot of money and lots of loyal henchman wants to take over the world by mass-producing vast quantities 2-3ft tall spider robots (SR).
The plan would begin with the EG distributing around a thousand shipping containers around the globe that contain machinery that can 1) outfit another shipping container with a copy of itself and 2) Can outfit another shipping container to become a mini-factory for producing the SR units.
The battle for world domination would begin by securing raw materials needed for SR production, and cranking up production so that targets could be overwhelmed by tens/hundreds of thousands of SR units.
Ultimately, the EG needs to make the production sites safe have a secure source of power to keep the manufacturing going. This requires a believable power source that could be packed into a shipping container but still crank out enough power to melt down any available steel for use in SR production.
Essentially, I'm trying to close the plot hole as to why authorities can't simply blackout the power grid to stop new SR units from being produced to stop the invasion. Plus, it would give the ability to manufacture SR units in places that don't have power grids.
What would be believable tech for shipping container-sized power generators that could be produced en masse without being detected prior to the trap being sprung? (This probably rules out miniature nuclear reactors, since they would be detected before they could get distributed around the world.) Ideally, it should be able to run >3 weeks with fuel initially contained in the shipping container.
FWIW: A standard 20′ shipping container can hold 1,170 cubic feet and the max gross weight is 30T