Update beaker-docker requirement from ~> 1.0 to ~> 2.1

Updates the requirements on [beaker-docker](https://github.com/voxpupuli/beaker-docker) to permit the latest version.
- [Changelog](https://github.com/voxpupuli/beaker-docker/blob/master/CHANGELOG.md)
- [Commits](voxpupuli/beaker-docker@1.0.0...2.1.0)

---
updated-dependencies:
- dependency-name: beaker-docker
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile

@@ -17,7 +17,7 @@ group :system_tests do
   gem 'beaker', '~> 4.0'
   gem 'beaker-rspec', '~> 8.0'
   gem 'beaker-puppet', '~> 1.21'
-  gem 'beaker-docker', '~> 1.0'
+  gem 'beaker-docker', '~> 2.1'
   gem 'beaker-puppet_install_helper', '~> 0.9'
   gem 'beaker-module_install_helper', '~> 1.0'
   gem 'voxpupuli-acceptance', '~> 1.0'