publish container image on ghcr.io

[renlord]

Now that dockerhub is imposing strict rate-limitting and curbing the availability of container images, it'd be best to also publish container images in other publicly accessible registry servers (i.e. ghcr.io)

The setup is quite simple and straightforward:

https://docs.github.com/en/packages/guides/migrating-to-github-container-registry-for-docker-images

[renlord]

🎉 thanks @ferferga