Make appu command build the docker image

[ifosch]

Currently we need to build the appu image before running the appu command.

Making this a default, or optional, behaviour for the appu command seems interesting. Adjust the README.md file accordingly. This could also imply remove the build and publish steps for the container, depending on the build time.

[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.