There are quite of few to choose from...
First off, will these cameras be placed right at the docks or will they reside inside of the warehouse? This will determine set of MiniDome cameras that can do the job...
RIGHT ON THE DOCK, MOUNTED ON THE EXTERIOR DOOR JAMB
Will the loading and unloading be taking place both day and night time?
BOTH DAY AND NIGHT
How are the lighting conditions in both cases (day and nigh)?
Will you be watching inside of the trailers or just what gets loaded or unloaded at the opening level?
AS FAR INSIDE AS POSSIBLE
Some of the answers to the above questions can provide some solid solution...