there are several types of shipping containers.
some (few) are "sealed". the integrity of the "sealed" ones is questionable, since they do get banged around a lot, and the seals on the doors and such are not well taken care of. generally speaking, the ones they sell are "pretty well sealed", and might be "good enough" for your needs.
most are vented. look for small plates spanning one corrugation near the top of the long sides, usually two or three in from each end. they are usually only three or four inches long. on the inside there are either holes drilled in those areas, or a square hole cut out, with a grill (usually poorly) installed. the grill is to keep rodents out, which it will, but not bugs.
i have several containers. if you are into this long term, and wish to protect the investment, here is what i did:
scrape down the area that the container is going to be resting. get some rocks from the local stone yard. chose what is cheap. around here, it was not gravel, it was round stone, about 1" in diameter. get enough to fill the area you scraped down. i went down a few inches, and about two feet longer and wider than the container (44' X 12'). i spread the stone so it was lower in the middle , and higher at the ends, from front to back.. if you swale it like this, make sure both ends are level with leach other, as well as level side to side.
go buy some 10' 4X6's, pressure treated if you can.
NOTE: containers come in many sizes, but only two "stacking" lengths. they are designed around either twenty or forty foot boxes. if you get a 28' container, it will have the stacking pin locations 20' apart, the additional eight feet will just hang over. same with a 53', the pin locations are 6.5'in from the ends. they are easy to spot. i bring this up because you really need to put supports under those stacking points. you might want supports near the ends too, depending on how you feel about the overhang, but dont just put the support at the ends of these odd sized containers.
place the 4X6's, with the 6" side down, at the support points on top of the stone. depending on how the delivery will be made, you might need to leave the wood at the "delivery end" until the truck is mostly out.
if you need to adjust things, and in my experience, you will, a floor jack will lift one corner of the container so you can adjust the wood. again, make sure it is all level
the rock will do several things. it will keep down the weeds, so they won't hold moisture against the box. it will keep dirt from splashing up onto the container when it rains, which is bad, because the dirt helps hold moisture against the box (notice a theme?).
if you are putting in more than one box, i suggest leaving a "person width" between them, so you can get in there if there are issues. you can always try and put them tight against each other, but i dont recommend it. it will be a lot of work getting them tight together, and they will tend to hold water between them, since they really won't be tight.
i put in the containers i have about 5 years ago. i had the seller paint them before bringing them over. (i paid about 2500$ each for 40' vented, "great condition" (no dents, completely working doors, no repairs to the sides having been made, etc) delivered about 30 miles. they were each delivered on a 30" Landoll trailer, so he could back in, and set the rear of the container on the 4X6 i had laid down, and pull forward/tip the trailer up to slide it off. worked well, only really had one issue (the second container was noticeably tipped to one side at the front, but not the back. after he left, i just jacked the container up, and used a tunneling bar to lift up the 4X6, and shoved rocks under it. it hasn't moved (noticeably) since.