Make appu command download the docker image

[ifosch]

Apparently, appy command is not downloading the appy container's image (ghcr.io/edyo/appu) when it's not present in the system.
We want to confirm this is correct and make appu command download the image in this case.

[ifosch]

This probably implies downloading the appu's Python code and Dockerfile. That might be unwanted or complicated. In any of these cases, we would prefer some other way.

Success in this story would mean we don't need #67, and probably we can omit the docker build and push from the build pipeline.