Create a Docker image

[ddundo]

I think we should create a Docker image with everything preinstalled so we can:

1. use it in GitHub Actions to run tests
2. Make it easier for people to get started with PyGEM

[ddundo]

@btobers could you please change the image's visibility to public? There's a "Package setting" on the right hand side menu in https://github.com/PyGEM-Community/PyGEM/pkgs/container/pygem

[btobers]

Yes, done.