Docker: ARG support fails

[hfhbd]

Sorry, the new docker ARG support #4598 still fails, because I forgot to update the file-updater...
The outdated docker version is parsed correctly, and the new version is found, but the file_updater is not able to update the new format: eg FROM $HUB/docker:20.10.7-dind to FROM $HUB/docker:20.10.12-dind

https://github.com/dependabot/dependabot-core/blob/main/docker/lib/dependabot/docker/file_updater.rb

/home/dependabot/dependabot-script/.bundle/ruby/2.7.0/gems/dependabot-docker-0.172.1/lib/dependabot/docker/file_updater.rb:57:in `updated_dockerfile_content': Expected content to change! (RuntimeError)
from /home/dependabot/dependabot-script/.bundle/ruby/2.7.0/gems/dependabot-docker-0.172.1/lib/dependabot/docker/file_updater.rb:25:in `block in updated_dependency_files'
from /home/dependabot/dependabot-script/.bundle/ruby/2.7.0/gems/dependabot-docker-0.172.1/lib/dependabot/docker/file_updater.rb:19:in `each'
from /home/dependabot/dependabot-script/.bundle/ruby/2.7.0/gems/dependabot-docker-0.172.1/lib/dependabot/docker/file_updater.rb:19:in `updated_dependency_files'
from ./generic-update-script.rb:232:in `block in <main>'
from ./generic-update-script.rb:192:in `each'
from ./generic-update-script.rb:192:in `<main>'

[jeffwidman]

Closing as a dupe of #2057 since we had to revert the whole thing for now, and the fix (including the note about still needing to extend the File updater) is re-staged in #4837.