How to cache Yarn/Node packages on the App Platform

I have a Rails/Yarn app that I’m trying out on the App Platform, which is working fine - both the nodejs and ruby buildpacks run:

4 of 5 buildpacks participating
heroku/nodejs-engine        0.5.1
heroku/ruby                 0.230.3
digitalocean/ruby-appdetect 0.0.2
digitalocean/procfile       0.0.3

Bundler appears to cache the ruby gems, which is excellent, as the install times (for everything except the first deploy) are very quick. I see:

-----> Loading Bundler Cache

and then all the gems install in a matter of seconds.

However, the same doesn’t appear to happen for Yarn/Node - there’s no equivalent “loading cache” line for Yarn, and I infer all the modules install from scratch, as they take some time on every deploy:

[5/5] Building fresh packages...
Done in 385.20s.

Is there a way I can cache the Yarn packages for faster deploys? (I’m also a little confused as to the App Platform/Heroku Buildpacks relationship - what elements of the Heroku Buildpacks apply here - is that the documentation I should be reading to try to find these kind of answers?)

Many thanks!