Docker GHA: Upload Docker images on release

[oxzi]

With this change, the Docker GitHub Action now creates multi-platform Docker images on releases and pushes those to Docker Hub. The changes were both inspired by the Docker defaults and https://github.com/Icinga/icinga-kubernetes/blob/main/.github/workflows/docker-image.yml.

Note: I am unable to check if the necessary environment variables are already set in this repository. This needs to be done by somebody with the sufficient privileges first. Furthermore, there is no project on Docker Hub so far, maybe it needs to be created first.

[julianbrost]

Note: I am unable to check if the necessary environment variables are already set in this repository. This needs to be done by somebody with the sufficient privileges first. Furthermore, there is no project on Docker Hub so far, maybe it needs to be created first.

Looks like someone set them an hour ago (maybe @lippserd, it roughly matches when he commented here 😅), so that should just work now.

[julianbrost]

main branch from this repo uploaded successfully: https://hub.docker.com/r/icinga/icinga-notifications/tags (job: https://github.com/Icinga/icinga-notifications/actions/runs/10074220195/job/27849849365)

I created a fork to test the release workflow with the merged version, of course fails during docker login, but it would set the correct tags:

Run docker/metadata-action@v5
Context info
Processing images input
Processing tags input
Processing flavor input
Docker image version
Docker tags
  icinga/icinga-notifications:0.1.0
  icinga/icinga-notifications:0.1
  icinga/icinga-notifications:latest