I made one. The repo is here: https://github.com/hoijui/nim-ci
It contains the basic things required for Linux (native, 64bit) and Windows (cross, 64bit) compiling of Nim software.
I personally use it for GitLab-CI. It probably makes no sense for GitHub CI, but should be useful for anything using docker as a base. The README also contains a sample GitLab-CI script, and a link to a sample project using the docker image in its CI script.
Please give feedback (optimally in the form of issues on the GitHub repo above).