What is the best type of paint to paint a shipping container?
Nononono! Tell him to save his latex paint!

I sometimes paint things at industrial sites.  Latex works fine on the buildings, but when it is used on steelwork of any kind, it turns into an unmitigated disaster, that you pay for for decades. 

This is a metal thing left outdoors.  It will have much higher extremes of temperature than a house's walls.  It will also have thermal expansion unlike a house.  There is also an adhesion issue with the { oil-base, possibly LPU } paint that was applied in the factory (in east Asia where nobody cares about enviro regs). 

Latex has managed to win on architecture because it's been engineered for 50 years to be a good fit.  Other than that, the only reason anyone even has a conversation about waterborne paints is to "spare the air".  But it doesn't spare the air to paint a bodge job that fails, is reapplied shortcut style, fails again and ultimately must use harsh chemicals to totally remove for a proper coat. That's like those early "low flow" toilets you had to flush 5 times.  

The greenest thing you can do is lay down paint that will last.   Here's how: 


Given this thing's long saltwater career, I would hose it down good and plenty while giving it a good old "carwash" style wash.  Salt also contaminates paint.  
Remove any identifiable rust either with a wire wheel or sandblast down to SSPC-SP10 near-white metal.  Get primer on it ASAP.  See below. 
"Scuff sand" every surface you want the paint to not fail. (I.e. Every surface). Scuff sanding means knocking the gloss of the previous layer, to create microscopic "jagged mountains" for the new primer/paint to lock in to.   Anywhere you blow through to bare metal, prime soon. 
wash off all contaminants, by optional carwash style washing, then solvent wipedown - the latter preferably the 2-cloth method.  Straight paint thinner is the classic, but they make low-VOC versions for Spare the Air territories that are terrible paint reducer but a fine wipedown solvent. You don't want soap residue or mineral residue from your city water (yes, it has minerals; if it didn't, it would leach them from pipes). 
Prime anywhere bare metal is exposed, e.g. Where you removed rust or scuff-sand blew through on any corners etc.  Sandblast or blow-through is excellent "prep" and you can use all sorts of primers.  Wire-wheel is less effective prep, and there, use an excellent over-rust primer such as ordinary Rustoleum 7769. 


Don't turn your back on bare metal.  It will rust in a couple of hours and compromise your primer's performance.  


If your topcoat requires a compatible primer, or if you're worried about topcoat compatibility, then you prime the entire container.  For instance, I like LPU topcoats, but they require a barrier coat of epoxy primer underneath. 
Finally, your topcoat:  Normally I roll-and-tip because I hate the PPE (masks, hoods) and extreme masking required for spray.  However containers are very irregular and don't need much masking, so it might be worth setting up for spray.  


For me, if this were a quickie, I'd have a conversation with a Sherwin Williams Industrial dealer about their best 1-part alkyd (oil) urethane.  If I were serious, it'd be LPU all the way.  My question is, "Do I ever want to paint this thing again?" 

I mention LPU, this is a 2-part (A and B part, like an epoxy) which combines a urethane A-part with an isocyanate B-part. (If this sounds scary: exactly.) The isocyanate and urethane splice to each other forming a poly-urethane molecule that is extremely long - far longer than can be obtained from a 1-part polyurethane paint, where the molecules are short.  This creates an extremely tough finish.  Any 2-part paint such as an automotive paint will be something like this, it's also used for aviation and marine. 

LPU and other 2-parts are safe as houses once it cures, and the isocyanate (or BPA) in the B-part will stay in the resin and not evaporate by itself, so it's safe to brush (don't get it on your skin).  Spraying it is a whole 'nother kettle of fish.  You're turning it into a fine mist, and you must not breathe that! At all! The factory recommends an external air supply.  I prefer marine paint since it is less encumbered by environmental regs, cheaper and made to brush. It's perfectly appropriate; it's a marine container after all. 

This safety issue is why 2-part paints aren't marketed to consumers, except for garage floor paint which is always rollered.