Trigger `covidcast-docker` image build automatically

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.

Start by reading #279 and reviewing the linked covidcast repository alongside this repository. Determine where the workflow should live and how a merge into the evalcast branch can trigger the covidcast-docker build and push to ghcr.io. Done means the image is built automatically after the specified merge with the required credentials.

Written by the indexing model from the issue text.