Prebuild Docker image to improve build time

[frankie567]

Follow-up of the discussion in #29.

Summary of the changes:

• Adds a new workflow build.yaml that builds and pushes a Docker image to the GitHub registry when a release is published
• action.yml now uses the prebuilt image instead of the Dockerfile
• In the workflow upgrader.yaml, we take care of replacing the tag in action.yml so it uses the newly tagged image
• In test.yml, before running the test, we manually replace the image path by Dockerfile
  • It's needed to be sure we are testing the latest changes. Otherwise, we would have issues because it could try to test an image that hasn't been built yet.

To make it work on you repository, you'll need to enable the improved Container registry on your GitHub organization: https://docs.github.com/en/packages/working-with-a-github-packages-registry/enabling-improved-container-support-with-the-container-registry#enabling-the-container-registry-for-your-organization-account

I hope it'll be helpful! Feel free to suggest improvements 😄

[exelban]

Hi. Looks nice. It's a lot of changes. And I don't have now free time to take a look( But I will try to check it the next week.

[exelban]

Hi. I finally find the time. Looks great, thanks for your contribution.