Feature: `bundle install --bundler` to install version required by Gemfile.lock/gems.locked

[svoop]

Dockerizing my pet project apps on top of Ruby images, I'm often seeing this while building images:

Warning: the running version of Bundler (2.1.4) is older than the version that created the lockfile (2.2.4). We suggest you to upgrade to the version that created the lockfile by running gem install bundler:2.2.4.

This mostly is no big deal, at least if the Ruby image is based on the same Ruby version as where the gems.locked/Gemfile.lock was created. However, it would be nice to install the correct version of Bundler before the actual bundle.

Currently, the Bundler version has to be extracted from gems.locked/Gemfile.lock by use of external commands which the image must feature:

RUN bundle config set without 'development test' && \
  version=$(grep -iA1 "bundled with" gems.locked | tail -n 1 | awk '{$1=$1};1') && \
  (if [ -n "$version" ]; then gem install bundler:$version; fi) && \
  bundle install --jobs 20 --retry 5

How about teaching Bundler a new trick to do the same?

RUN bundle config set without 'development test' && \
  bundle install --bundler && \
  bundle install --jobs 20 --retry 5

What are the chances for such an enhancement to be merged?

[deivid-rodriguez]

Hello @svoop! Thanks for the suggestion.

In few words, that's how I think bundle install should work by default, I don't think we even need a special flag.

I plan to get this and other related issues sorted out for the next minor bundler release. For the moment we have this WIP PR: #4076.

[deivid-rodriguez]

I'll close this since we plan to do this, but by default, and I got no further feedback here.