I have always used amenity=loading_dock as nodes on buildings. My only question is, do you add some type of line for the individual lanes? What if there are 4 loading dock lanes, then a loading ramp, then 4 more loading lanes? What’s the proper procedure for mapping such lanes, if at all?
Have you got the loading-dock node marked on the ramp? In that case, probably a service road each side of it, tagged as 4 lanes & access=private (=destination maybe?).
Alternatively, one of 8 lanes would also work.
For, me it depends on how many lanes. And how lazy I am feeling. :) Many of the examples below are from 3 site surveys that I did on foot totalling about 10 hours (covering Greenogue Business Park and Aerodrome Business Park), using a non-GPS camera, and whatever street-level photos are available via OSM.
A big issue is that some locations might have lots of truck space, but not every truck space has a loading dock. Having street-level photography can be useful in such cases. Loading docks might be separated by a large space from the next loading dock or even by walls or buildings. There may also be ramps for forklift trucks and small vehicles, so careful checking of shadows can be useful - see here: Node: 13394251625 | OpenStreetMap on
Esri World Imagery.
From a business perspective, having adjustable heights and the correct approach slopes can be important - roll goods downhill off big truck, roll goods downhill onto van.
The simplest loading dock has space for only one truck: Node: 11888359252 | OpenStreetMap amenity=loading_dock goes on the building and noexit=yes on the end of the highway=service. Otherwise, QA software may complain about unconnected ways / nodes. It would be nice to be able to have amenity=loading_dock treated like entrance=main by the QA software, and act like a noexit=yes
Here: Node: 10133202237 | OpenStreetMap I mapped one out of about 20. The new Esri World Imagery actually shows how many loading docks there are. highway=service is marked as cutting=yes, although it’s rather arguable in cases like this, with so many lanes. Note retaining walls. 
Here: Node: 6297858333 | OpenStreetMap there appears to be two pairs of loading docks.
Here: Node: 11867191623 | OpenStreetMap I’ve mapped two out of about 10.
For this: Way: Amazon (1151332482) | OpenStreetMap there was a strong “Nope, not mapping all of them.”, as that would be over-mapping. There are about 30 loading docks and mapping every single lane approaching a loading dock would not make sense. I would have mapped one highway=service and added lanes=*.
Here: Way: 1296174894 | OpenStreetMap I did map lanes=14. If you look at Bing Maps Aerial, they are even number 1-14.
Potential pitfall: mapping 20 loading bays meticulously and then realising there are 19 or 21 or 10 or 100+: Way: Tesco Distribution Centre (199650602) | OpenStreetMap (this is one of the largest buildings in the world, by volume)
You have provided very good information. I will have to think how I will map this. Either one line with the number of lanes, or just add nodes of the docks.