You wouldn’t fire from inside the port because it’d be pointless. Just bring over a SADM equivalent in a container, set the timer and leave.
Container launched ballistic missiles are made so nobody can track your missile carriers and you can fire from innocuous container ships at sea.
You’d buy the ports to bring in troops and equipment.
Except our tracking systems are looking over the horizon, not originating in CONUS to a CONUS target.
It’s like using a nuclear sniper behind lines.