--without development,test being ignored on bundle install

[stuartluscombe]

Issue:
When deploying a Rails app with the Ruby 2.6.2 image I'm finding that gems I have within the development and test groups of my Gemfile are being installed despite having the application setting BUNDLE_WITHOUT set.

At the point of pushing my code to the azure remote and the build starts it does say it's running it with --without development,test, but I can clearly see that the gems are being fetched and installed.

Gemfile:

group :development do
  # Access an interactive console on exception pages or by calling 'console'
  # anywhere in the code.
  gem 'listen', '>= 3.0.5', '< 3.2'
  gem 'web-console', '>= 3.3.0'
  # Spring speeds up development by keeping your application running in the
  # background. Read more: https://github.com/rails/spring
  gem 'spring'
  gem 'spring-watcher-listen', '~> 2.0.0'

  # Use Solargraph for Intellisense, linting etc.
  gem 'solargraph'

  # Add rubocop-rails to get rubocop working nicely with Rails.
  gem 'rubocop-rails'

  # Better errors
  gem 'better_errors'
  gem 'binding_of_caller'
end

Deploy output:

...
remote: Bundle install with options --without development,test
...
remote: Fetching better_errors 2.5.1
remote: Installing better_errors 2.5.1
...
remote: Fetching solargraph 0.38.5
remote: Installing solargraph 0.38.5
...
remote: Gems in the group development,test were not installed.
...

Steps to reproduce:

1. Set up a new Ruby on Rails app and add some gems to the development and/or test sections of the Gemfile.
2. Set up a new Web App using the Ruby 2.6.2 image
3. Add the BUNDLE_WITHOUT setting to exclude "development,test" (as per the MS documentation)
4. Push the app to Azure.