Consider merging the `covidcast-docker` repo into this repo

The `covidcast-docker` repo is an important part of the forecast eval package.

- The repo is very small
- Because it is separate from the rest of the the forecast eval code, is difficult to discover (or remember!).
- The local docker file [uses the `covidcast-docker` docker image as-is](https://github.com/cmu-delphi/forecast-eval/blob/f12ab6f303ba81d6cbc32d61720061474496a00f/docker_build/Dockerfile)
- As far as I'm aware, it is not used by any other projects despite the very general-sounding name.

It seems that there is no reason to have the `covidcast-docker` repo separated from the rest of the forecast eval code; this setup introduces unnecessary complexity.

However, this deserves a bit more research into the historical decision to separate the repositories, and to make sure no other tools, apps, etc use the `covidcast-docker` docker image.

Start with docker_build/Dockerfile and the covidcast-docker repository to understand how the image is used. Research the historical reason for the separate repository and check whether other tools or applications consume the image. Done means documenting the findings and establishing whether a safe repository merge is possible.

Written by the indexing model from the issue text.