Caching tailwind build assets in CI

[searls]

I'm here with a simple question: "is it possible to cache the tailwind build in CI?" My brand new app's Tailwind 4 build is quite slow in GitHub Actions and I would like to find a way to avoid paying the piper on every build (actually, twice on every build).

I was just scrutinizing my build time in GitHub Actions and came across this:

$ bin/rails db:test:prepare test test:system
≈ tailwindcss v4.1.4

Done in 9s
Running 47 tests in a single process (parallelization threshold is 50)
Run options: --seed 32133

# Running:

...............................................

Finished in 6.400974s, 7.3426 runs/s, 36.0883 assertions/s.
47 runs, 231 assertions, 0 failures, 0 errors, 0 skips
≈ tailwindcss v4.1.4

Done in 8s
Running 1 tests in a single process (parallelization threshold is 50)
Run options: --seed 604
# Running:
Capybara starting Puma...
* Version 6.6.0, codename: Return to Forever
* Min threads: 0, max threads: 4
* Listening on http://127.0.0.1:39819/
.
Finished in 2.022590s, 0.4944 runs/s, 0.4944 assertions/s.
1 runs, 1 assertions, 0 failures, 0 errors, 0 skips

Because both rails test and rails test:system each precompile assets (something I feel like is a bug when the same process is invoking them), I'm getting hit with a particularly expensive compilation step of 9s and then another of 8s. So that's 17 seconds for a test suite that doesn't depend on styles. It stood out to me because locally each build takes on the order of 380ms.

I was thinking that build results might be cached someplace like tmp/cache, but it appears they aren't cached anywhere in development or test? @flavorjones could you shed any light on what my options might be?