Build docker image in a github workflow

[amatissart]

The docker image is pushed to Github Container Registry (https://ghcr.io).

The login uses the GITHUB_TOKEN provided automatically during the workflow run.
See https://docs.github.com/en/actions/reference/authentication-in-a-workflow for more details.

[amatissart]

@remi-dupre The docker image is now available here: https://github.com/osm-without-borders/cosmogony/pkgs/container/cosmogony