Limiting DLC size / forcing pruning of DLC

Over time the amount of data in our docker layer cache grows to the documented limit (~15GB). Is there any way of lowering this limit or preemptively pruning the size of the cache?

At this point environment spin up time is typically around 90s, but if I delete the cache and run a build or two then environment spin up time drops to about 20s (but will get progressively slower over time) & the nature of our usage is such that I still get all the benefit of the DLC (we only build images from the main branch - it would be very unusual for layers other than those in the most recent build or two to get reused)