There is a feature in the newest version of Bundler (2.4) that speeds up installation of gems with git sources. In previous versions it would clone the full repository and with a big repo like Rails it could take a while (~30 seconds). Now it is much faster and we can start a new application from the as-yet-unreleased version of rails by using the rails repo as our gem source in less than a minute.
Jump into a docker ruby environment
docker run -it ruby:latest bash
Update bundler to the latest version (2.4 at the time of this writing)
gem update bundler
Add rubygems source and the rails gem straight from Github to a Gemfile
echo "source 'https://rubygems.org'" >> Gemfile echo "gem 'rails', github: 'rails/rails'" >> Gemfile
Start a new rails app using the version we just installed using bundler.
bundle exec rails new testing --dev
You can read more about Bundler 2.4
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