RUN MAKE="make -j$(nproc)" bundle install && \

This will parallelize compilation for gems with native extensions.

I tested this with our Rails app on my laptop (Macbook M3 Pro):

• default (no parallelization): ~112 seconds for the bundle install step
• with 2 CPUs: ~85 seconds
• with 4 CPUs: ~64 seconds

We could make this configurable to play it safe, but I don't think there's much upside to not doing this.

(Also, bundle has its own --jobs argument, but it didn't seem to make any noticeable difference when I tried it. I assume the GVL prevents bundle from being able to truly parallelize work.)