Docker: You must use Bundler 2 or greater with this lockfile

Question

I am trying to install bundler version 2.1 in a docker image that is built from ruby:2.4.1. My Docker file looks the following way:

FROM ruby:2.4.1

RUN \
  gem update --system --quiet && \
  gem install  bundler -v '~> 2.1'

# Other commands

But when I try to run bundle install it fails with

You must use Bundler 2 or greater with this lockfile.

When I run inside a container gem info bundler it outputs:

bundler (2.1.2, 1.15.4)
    Authors: André Arko, Samuel Giddins, Colby Swandale, Hiroshi
    Shibata, David Rodríguez, Grey Baker, Stephanie Morillo, Chris
    Morris, James Wen, Tim Moore, André Medeiros, Jessica Lynn Suttles,
    Terence Lee, Carl Lerche, Yehuda Katz
    Homepage: https://bundler.io
    License: MIT
    Installed at (2.1.2): /usr/local/bundle
                 (1.15.4): /usr/local/lib/ruby/gems/2.4.0

I tried the following fixes:

# Set bundler 2.1.2 as default
bundler config default 2.1.2

# Update bundler
gem update bundler

But they didn't work. The system continues to use bundler v1.15.4

How can I make bundler v2.1.2 as default inside a ruby docker image?

Answer 1

I was able to switch to bundler version 2.1 in my docker image using the following commands:

# Dockerfile
RUN \
  gem update --system --quiet && \
  gem install  bundler -v '~> 2.1'
ENV BUNDLER_VERSION 2.1