publish docker image to GitHub Container Registry

[X-Wei]

cf. https://docs.github.com/en/free-pro-team@latest/actions/guides/publishing-docker-images#publishing-images-to-github-packages

[X-Wei]

To provide the secret actually we need to add the token in the repo's secrets settings: https://github.com/X-Wei/pelican-gh-actions-xwei/settings/secrets