Optimise CI

[dacook]

There are potential ideas to reduce CI resources and runtime:

name	potential savings	effort
Cache assets for precompilation (see openfoodfoundation/openfoodnetwork#12580 (comment))		high
Cache node_modules folder (openfoodfoundation/openfoodnetwork#12581)	5 seconds	medium
Parallelise setup tasks		high

• might be able to set up db without Bundler
• can run bundler and yarn in parallel
• can run Webpacker and Sprockets compilation in parallel too?

Context

This was triggered when trying to solve a flaky spec by precompiling sprockets assets: openfoodfoundation/openfoodnetwork#12552
It has now been worked around, but we may need to investigate again one day.

[dacook]

I'm just organising these ideas here so we can move on. I don't think there's high value in investigating it right now.

As much as I'm tempted to micro-optimise, we should avoid doing anything unless there's a pre-built solution.