Bundler 2 is not installing inside docker - ruby-on-rails

Question

I'm trying to install bundler v2 in docker file and it installs successfully. when i run rubocop i get below error.
warn_for_outdated_bundler_version': You must use Bundler 2 or greater with this lockfile. (Bundler::LockfileError)
which still seem to think that I am using bundler older version, how can I fix this error
Dockerfile is
FROM ruby:2.3.4-alpine
WORKDIR "/var/www/app"
COPY Gemfile Gemfile
COPY Gemfile.lock Gemfile.lock
RUN gem install bundler && bundle install
bin/bundle file is
require "rubygems"
m = Module.new do
module_function
def invoked_as_script?
File.expand_path($0) == File.expand_path(__FILE__)
end
def env_var_version
ENV["BUNDLER_VERSION"]
end
def gemfile
gemfile = ENV["BUNDLE_GEMFILE"]
return gemfile if gemfile && !gemfile.empty?
File.expand_path("../../Gemfile", __FILE__)
end
def bundler_version
#bundler_version ||= begin
env_var_version || cli_arg_version ||
lockfile_version || "#{Gem::Requirement.default}.a"
end
end
def load_bundler!
ENV["BUNDLE_GEMFILE"] ||= gemfile
# must dup string for RG < 1.8 compatibility
activate_bundler(bundler_version.dup)
end
def activate_bundler(bundler_version)
if Gem::Version.correct?(bundler_version) && Gem::Version.new(bundler_version).release < Gem::Version.new("2.0")
bundler_version = "< 2"
end
gem_error = activation_error_handling do
gem "bundler", bundler_version
end
return if gem_error.nil?
require_error = activation_error_handling do
require "bundler/version"
end
return if require_error.nil? && Gem::Requirement.new(bundler_version).satisfied_by?(Gem::Version.new(Bundler::VERSION))
warn "Activating bundler (#{bundler_version}) failed:\n#{gem_error.message}\n\nTo install the version of bundler this project requires, run `gem install bundler -v '#{bundler_version}'`"
exit 42
end
def activation_error_handling
yield
nil
rescue StandardError, LoadError => e
e
end
end
m.load_bundler!
if m.invoked_as_script?
load Gem.bin_path("bundler", "bundle")
end
and the bundler version inside Gemfile.lock is 2.0.2

Answer 1

set BUNDLER_VERSION environment variable inside Dockerfile:
ENV BUNDLER_VERSION=2.0.2
or if you have docker.env:
BUNDLER_VERSION=2.0.2

Answer 2

I faced this problem also but as well as defining the BUNDLER_VERSION environment variable, I also needed to install that version into my container.
My Dockerfile therefore would look like:
FROM ruby:2.3.4-alpine
WORKDIR "/var/www/app"
ENV BUNDLER_VERSION=2.0.2
COPY Gemfile Gemfile
COPY Gemfile.lock Gemfile.lock
RUN gem install bundler -v "$BUNDLER_VERSION" && bundle install