In anything, the reason I store such flammables outside my shops container or otherwise.
I recently bought some industrial CO2 fire extinguishers for the cylinder carts but they're full (and cheap to refill) so now one sits just inside the man door I added to my motorcycle garage High Cube. I can open the valve (I safety wired the trigger paddle to the open position) and inert the entire box.
This. The haters don't know how to do it properly so they assume it cannot be done (commercial and military users prove otherwise) but the dreamers are just as bad because they fucking refuse to learn how to find simple information.
Look at the bottom of a container. There is no benefit to wanting one as a basement. Round culvert tubes as used by Ukraine are OTOH ideal for bunkers and the method using "elephant iron" piecemeal building proven in WWII.
The smart way is weld them side-by-side then taking advantage of your instantly useable space do the rest at leisure. It's easy. I positioned all my containers single-handed using jacks, rollers, cribbing, shims, steel cable and my Wyeth-Scott hand puller.
Weld the corner fittings after matching their height.and pulling the boxes together so there's no gap. Have half a sheet of 1/8" or thicker steel sheared to 2" wide strips to join the rest by bridging the gap. Tack the high side, finish weld the high side, beat the low side down then tack and finish weld that with FCAW (suitcase feeders are worth owning for this). I used up old .035" wire then switched to .045" which is a bit thick but was nearly free. Some care to avoid blowing through is needed so I start the welds on the container flowing the puddle to make the joint. Not hard but noobs may not think of that.
Using two pieces of angle bolted together after coating with sealant where they mate would permit welding and permit unbolting if you anticipate moving the structure later or separating the boxes to bridge with trussed roofing etc.