While there are various models of shipping containers, most of them are not airtight by design to allow circulation of air and avoid condensation and temperature build-up. They have seals to prevent water though.
Really?! I know that these things are built to be very stable, but they never gave me the impression to be air-tight. Not bad. I'm sure the world of shipping containers is a huge rabbit-hole to read into.
You don't, they cannot be opened from inside once locked. Also they're airtight, so bang on the walls and hope help arrives before you suffocate.
That's more nightmare inducing than I was hoping.