Fix Github Actions workflow for publish_docker

[pcallewaert]

Hi 👋

This PR fixes 2 issues with the current workflow:

• Use v4 of the checkout action to remove the deprecation warnings about nodejs12
• Fix a typo in the output of the publish-rubygems-action. In the action it uses new_version as output (src: https://github.com/discourse/publish-rubygems-action/blob/v2/entrypoint.sh#L25) but we were checking on new_version_published. This meant the if statement for the publish_docker job was always false.

It's not that easy to test, but I tested this out on my fork (with some workarounds) and the publish_docker job triggered now successfully: https://github.com/pcallewaert/prometheus_exporter/actions/runs/7637020510

This should fix #301

[sosedoff]

Interesting. When i worked on the release flow i also used my own fork for testing, and it was working initially. Suspecting there were slight changes in the action that we missed since no new versions of this gem were published for a while (about a year?).

[SamSaffron]

I am more than happy to try it out!