Hi,
If you look at several of the photos (especially the side one, about half way through the article) you will see numberous stacks of large wooden blocks spaced to fall along the main webframes of the ship all along the keel. In addition you can also see additional "side block" stacks locaterd to port and starboard away from the center keel, in the 2nd and 3rd images in the article.
The total number of blocks is calculated for the ship, based on its likely worst case loading and the crushing limits of the wood used, among other things. In general though, the surface area of all those block towers are enough to support the entire weight of the ship, and with them located under the main keel and webframes of the ship the pressure forces are distributed throughout the ships main structure when sitting on the blocks. 
PF
PS. For reference here is an image of a different drydock, showing similar block stacks for some other ship, prior to the dock being flooded and the ship being brought in.
[Source: https://www.royalhaskoningdhv.com/nl-nl/nederland/diensten/diensten-van-a-tot-z/dry-docks/5944 ]