[13.12] Use a stable version of docker instead of docker:latest

This should fix the error technical-writing#533 (comment 818612745).

`/root` is not writable.
Bundler will use `/tmp/bundler20220122-1-5zons41' as your home directory temporarily.
Fetching gem metadata from https://rubygems.org/..........
Fetching rake 13.0.6
Installing rake 13.0.6
Gem::FilePermissionError: You don't have write permissions for the
/usr/local/bundle directory.
An error occurred while installing rake (13.0.6), and Bundler cannot continue.
Make sure that `gem install rake -v '13.0.6' --source 'https://rubygems.org/'`
succeeds before bundling.
In Gemfile:
  rake
The command '/bin/sh -c NOKOGIRI_USE_SYSTEM_LIBRARIES=1 bundle install --jobs 4' returned a non-zero code: 5