Fix Dockerfile to match Gemfile.lock

Gemfile.lock requires 1.16.6. But gem install bundler installs the latest version.

Without this change the pipeline will break at first step.

 /usr/lib/ruby/2.5.0/rubygems.rb:284:in `find_spec_for_exe': Could not find 'bundler' (1.16.6) required by your /Gemfile.lock. (Gem::GemNotFoundException)
To update to the latest version installed on your system, run `bundle update --bundler`.
To install the missing version, run `gem install bundler:1.16.6`
	from /usr/lib/ruby/2.5.0/rubygems.rb:303:in `activate_bin_path'
	from /usr/local/bin/bundle:23:in `<main>'

See https://gitlab.com/gitlab-org/creator-pairing/new-android/-/jobs/530246091