automatically build and upload docker image

[danshapero]

Right now we're using a Docker image that I manually built and pushed to the github container registry (some helpful instructions on how to do that). The dockerfile lives in the ci/ directory. We may need to update that, for example for pygmsh-7.0. I should add another github workflow to rebuild and push that docker image to GHCR. We can make this trigger only on changes in the ci/ folder, see this and this.