add debian:buster-slim based docker image

[rgl]

This add a github actions workflow to build the multi-platform container image and publish it to docker hub (only when there is a v prefixed tag associated with the commit).

In order to publish to docker hub a couple of secrets need to be created in this repository settings:

1. DOCKER_HUB_USER
2. DOCKER_HUB_ACCESS_TOKEN: this can be created from your docker hub security page inside you account.

It probably would also be nice to include a badge in the readme, something alike:

Also it might be useful to modify the README to mention the docker hub image?

Also maybe we should include the edid file and load it everytime the container is started? this would mean that the v4l2-ctl tool (and dependencies) would have to be included in the container image. Hummm, but this makes the assumption that the user is using the hdmi-to-csi bridge, so maybe not such a great idea.

This should close #32.