Trigger `covidcast-docker` image build automatically

[nmdefries]

Following #279, when a PR is merged into the evalcast branch of https://github.com/cmu-delphi/covidcast, use a workflow to automatically build a new covidcast-docker docker image.

I'm not sure how complex inter-repo workflows are. It would be nice to have the workflow in this repo so it's easier to keep track of, but it is probably easier to set up the workflow in the covidcast repo. The build image is pushed to the ghcr.io image repository, which is accessible from anywhere with the right credentials.