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

[nmdefries]

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
• 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.