Maybe put a raised platform at the edge of the tarmac the same height as the floor of the cargo bay. Back up to the thing and roll the cargo off so you don't have to worry about aligning two different vehicles. One side of the platform could slope to match the height of the truck too. Very easy to implement.Also the Lockheed "Flatbed" concept of the 1980's: Lockheed Flatbed - Wikipedia
But no, rather than a detachable pod (although that might work very well with intermodal containers!), I was thinking of something much more like the C-119 or any of the 1950's cargo airplanes with noses that swung completely aside. The airplane's gear would be height-adjustable to match the bed-height of the truck delivering or picking up the standard cargo containers, and you just roll-on/roll-off directly between truck and aircraft. Put a couple of cargo-bay buffers on the lower lip of the airplane cargo bay and, with the nose or the rear full-width doors wide open, just back the truck right up to the airplane.
Something like you describe could be an airplane that would roll right over an intermodal container on a truck, winch it up against the bottom of the aircraft between the booms, clip on a nose cone and tail cone, and head for the runway. You'd have a drag penalty from the ribbing on the side of the container, but the convenience and quick-turn might be worth the extra fuel.