Add GitHub Action setup for building client container image
Add GitHub Action setup for building client container image

The action uses steps defined by docker to build and push a new image.

Action runs periodically (every Tuesday early morning) and it also has
a manual trigger for manual runs.

It builds container images for amd64 (64-bit Intel or AMD CPUs) and
also 64-bit ARM (mainly for Macbook M1).

The container image is pushed to the GitHub Container Registry (ghcr.io).

The images built with my account can be found under
ghcr.io/filbranden/vimgolf:latest

(Once this commit is merged to igrigorik's repository, the Action should
start creating containers under his account as well.)

Tested:

```
$ podman run -it --rm -e key=... ghcr.io/filbranden/vimgolf 9v0062e3e96a000000000226
```

igrigorik/vimgolf,igrigorik/vimgolf,igrigorik/vimgolf,igrigorik/vimgolf