Allow optional caching in failed builds

Description

It's possible that you have a configuration where the outcome of the pipeline has nothing really to do with the quality of the cache. For example, you may be syncing quite a lot of NPM dependencies that are otherwise unchanged by your code. Throwing this valuable cache away because your own code failed to build is not an efficient use of time and resources.

This is particularly exacerbated when getting an initial pipeline working and nothing is ever cached until your first green checkbox shows up.

Proposal

Just like it's possible to specify artifacts:when = always, it should be possible to specify cache:when = always.

Links / references

Documentation blurb

In the documentation we should highlight this as a way to speed up pipelines when you're caching external dependencies, and also mention it in the efficiency docs.

Overview

What is it? cache:when in gitlab-ci.yml Why should someone use this feature? To save time on failed builds. What is the underlying (business) problem? Productivity. How do you use this feature? Add cache:when = always to gitlab-ci.yml

Use cases

Save time on subsequent failed builds.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

• Feature assurance
• Documentation
• Added to features.yml