Jekyll: sass-listen-4.0.0 revealed dependencies not in the API or the lockfile

GitLab CI/CD
1

I can successfully build and run jekyll but on gitlab the build job fails with

Downloading sass-listen-4.0.0 revealed dependencies not in the API or the
lockfile (rb-fsevent (~> 0.9, >= 0.9.4), rb-inotify (~> 0.9, >= 0.9.7)).
Either installing with `--full-index` or running `bundle update sass-listen`

I am a little confused and the results returned by google did not helped me further - perhaps for my lack of ruby knowledge - but the error message is not very clear to me. Apparently if I check the Gemfile.lock the dependencies seem to be fulfilled:

GEM
  remote: https://rubygems.or0g/
  specs:
	addressable (2.5.2)
	  public_suffix (>= 2.0.2, < 4.0)
	colorator (1.1.0)
	ffi (1.10.0)
	forwardable-extended (2.6.0)
	jekyll (3.5.0)
	  addressable (~> 2.4)
	  colorator (~> 1.0)
	  jekyll-sass-converter (~> 1.0)
	  jekyll-watch (~> 1.1)
	  kramdown (~> 1.3)
	  liquid (~> 4.0)
	  mercenary (~> 0.3.3)
	  pathutil (~> 0.9)
	  rouge (~> 1.7)
	  safe_yaml (~> 1.0)
	jekyll-paginate (1.1.0)
	jekyll-sass-converter (1.5.2)
	  sass (~> 3.4)
	jekyll-sitemap (1.2.0)
	  jekyll (~> 3.3)
	jekyll-watch (1.5.1)
	  listen (~> 3.0)
	kramdown (1.17.0)
	liquid (4.0.1)
	listen (3.1.5)
	  rb-fsevent (~> 0.9, >= 0.9.4)
	  rb-inotify (~> 0.9, >= 0.9.7)
	  ruby_dep (~> 1.2)
	mercenary (0.3.6)
	pathutil (0.16.2)
	  forwardable-extended (~> 2.6)
	public_suffix (3.0.3)
	rb-fsevent (0.10.3)
	rb-inotify (0.10.0)
	  ffi (~> 1.0)
	rouge (1.11.1)
	ruby_dep (1.5.0)
	safe_yaml (1.0.4)
	sass (3.7.3)
	  sass-listen (~> 4.0.0)
	sass-listen (4.0.0)
	  rb-inotify (>= 0.9.7, >= 0.9)

PLATFORMS
  ruby

DEPENDENCIES
  jekyll (= 3.5.0)
  jekyll-paginate
  jekyll-sitemap
  tzinfo-data

RUBY VERSION
   ruby 2.6.0p0

BUNDLED WITH
   2.0.1

My .gitlab-ci.ymllooks like this:

image: ruby:2.6

variables:
  JEKYLL_ENV: production
  LC_ALL: C.UTF-8

before_script:
  - gem update --system
  - gem install bundler:2.0.1
  - bundle install --full-index

pages:
  stage: deploy
  script:
  - bundle exec jekyll build -d public
  artifacts:
	paths:
	- public
  only:
  - master

Here the detailed error message

Running with gitlab-runner 11.7.0-rc1 (6e20bd76)
  on docker-auto-scale ed2dce3a
Using Docker executor with image ruby:2.6 ...
Pulling docker image ruby:2.6 ...
Using docker image sha256:f28a9e1d0449a75ac4baa10614da765e9ed09fc4478a6f71e9c4436234395af3 for ruby:2.6 ...
Running on runner-ed2dce3a-project-6029390-concurrent-0 via runner-ed2dce3a-srm-1547396261-bace68e7...
Cloning repository...
Cloning into '/builds/wyssmann/wyssmann.gitlab.io'...
Checking out 504ad855 as master...
Skipping Git submodules setup
$ gem install bundler:2.0.1
Successfully installed bundler-2.0.1
1 gem installed
$ bundle install --full-index
The dependency tzinfo-data (>= 0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for x86-mingw32, x86-mswin32, x64-mingw32, java. To add those platforms to the bundle, run `bundle lock --add-platform x86-mingw32 x86-mswin32 x64-mingw32 java`.
Fetching source index from https://rubygems.org/
Fetching public_suffix 3.0.3
Installing public_suffix 3.0.3
Fetching addressable 2.5.2
Installing addressable 2.5.2
Using bundler 2.0.1
Fetching colorator 1.1.0
Installing colorator 1.1.0
Fetching ffi 1.10.0
Installing ffi 1.10.0 with native extensions
Fetching forwardable-extended 2.6.0
Installing forwardable-extended 2.6.0
Fetching rb-inotify 0.10.0
Installing rb-inotify 0.10.0
Fetching sass-listen 4.0.0
Downloading sass-listen-4.0.0 revealed dependencies not in the API or the
lockfile (rb-fsevent (~> 0.9, >= 0.9.4), rb-inotify (~> 0.9, >= 0.9.7)).
Either installing with `--full-index` or running `bundle update sass-listen`
should fix the problem.
ERROR: Job failed: exit code 1

How can I fix the problem so that the build runs again in gitlab?