Optimise CI

[dacook]

There are potential ideas to reduce CI resources and runtime:

name	potential savings	effort
Cache assets for precompilation (see https://github.com/openfoodfoundation/openfoodnetwork/pull/12580#discussion_r1643934578)		high
Cache node_modules folder (https://github.com/openfoodfoundation/openfoodnetwork/pull/12581#:~:text=We%20could%20probably%20save%20a%20further%206%20seconds%20by%20caching%20node_modules.%20This%20guide%20looks%20straightforward.%20But%20why%20is%20this%20not%20built%2Din%20to%20an%20official%20action%3F%3F%0Ahttps%3A//github.com/actions/cache/blob/main/examples.md%23node%2D%2D%2Dyarn)	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: https://github.com/openfoodfoundation/openfoodnetwork/pull/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.