cache without installing

[akostadinov]

Hello, I want to speed up container image builds so my idea is to cache the gems, then use the cache during the build.

For this I need the cache to be of the non-installed gem files. Ideally it would cache files for all PLATFORMS listed in Gemfile.lock.

I saw a related issue #489 but it is about running in an image, not building one.

Also I saw #451 but it will not help when building multiple architectures. At least cache would need to be different for each. Also layers unnecessary increase image size.

[eregon]

Caching .gem files is out of scope of this action.
This action is meant to be used on runners directly, not inside a Docker image, and not to build a Docker image.

[eregon]

Also BTW caching .gem files is unlikely to provide any speedup, it's a download vs a few downloads.