Move Yarn to a separate image

[michaelbaudino]

Many people argued that they would prefer an image for the Ruby app and another one for the Yarn app, rather than this turnkey-but-overstuffed image. And they are right, at least because upgrading Yarn should not be mandatory (nor bound to a Ruby version).

It may be possible to extract the Yarn-related tools to another image (and run it in a separate Docker Compose service) while conserving the original goal of simplistic and reusable Docker configuration.

Thanks to @ylecuyer, @guille-moe and @GCorbel for their feedback and ideas 🎩 (you guys feel free to unsubscribe from this issue, I just mentioned you because you deserve the credits)