Bundles are not installed in `/vendor/bundle`, cannot launch server offline

[T3sT3ro]

When starting a jekyll/jekyll:4.0 (and 4.2.2) with volume ./vendor/bundle:/usr/local/bundle:Z, installed gems don't cache locally. They install into /usr/gem, so starting jekyll server without internet access is impossible.

I tried attaching to container and doing any combination of bundle cache, bundle package, bundle update, bundle install, bundle install --deployment, but nothing populates the vendor/bundle...

Here is my docker-compose.yml:

services:

  jekyll-server:
    image: jekyll/jekyll:4.2.2
    volumes:
      - .:/srv/jekyll:Z
      - ./vendor/bundle:/usr/local/bundle:Z
    ports:
      - '[::1]:4000:4000'
      - '[::1]:35729:35729'
    command: jekyll serve --livereload

  bundle-install: 
    image: jekyll/jekyll:4.2.2
    volumes:
      - .:/srv/jekyll:Z
      - ./vendor/bundle:/usr/local/bundle:Z # mount to update gems cache
    command: bundle update
    ```

[T3sT3ro]

I managed to make it (kind of?) retain it's gems, by using bundle config --local path vendor inside a container with mounted volume. This resulted in the addition of BUNDLE_PATH: "vendor" line inside the vendor/bundle/config.

Still, I think there should be some easier way to do that, and the image should be updated. My current Gemfile is

Gemfile

source "https://rubygems.org"

# Hello! This is where you manage which Jekyll version is used to run.
# When you want to use a different version, change it below, save the
# file and run `bundle install`. Run Jekyll with `bundle exec`, like so:
#
#     bundle exec jekyll serve
#
# This will help ensure the proper Jekyll version is running.
# Happy Jekylling!
gem "jekyll", "~> 4.3.1"

# This is the default theme for new Jekyll sites. You may change this to anything you like.
gem "minima", "~> 2.5.1"

# If you want to use GitHub Pages, remove the "gem "jekyll"" above and
# uncomment the line below. To upgrade, run `bundle update github-pages`.
# gem "github-pages", group: :jekyll_plugins

# If you have any plugins, put them here!
group :jekyll_plugins do
  # gem "jekyll-feed", "~> 0.17.1"
end

# Windows does not include zoneinfo files, so bundle the tzinfo-data gem
# and associated library.
install_if -> { RUBY_PLATFORM =~ %r!mingw|mswin|java! } do
  gem "tzinfo", "~> 2.0.5"
  gem "tzinfo-data"
end

# Performance-booster for watching directories on Windows
gem "wdm", "~> 0.1.1", :install_if => Gem.win_platform?