How to speed up ruby bundle gems fetch

Short answer: use https://github.com/geminabox/geminabox for caching gems on your own host, or in your local network.

I was using bundler’s vendor/cache in docker container, but with such improvement my container size grown up to 1,2Gb very quickly.

Supply following settings to your geminabox container
RUBYGEMS_PROXY=true

https://medium.com/magnetis-backstage/how-to-cache-bundle-install-with-docker-7bed453a5800