Update images to accept BASE_IMAGE build argument

[chbrandt]

Major update is the suuport for choosing the base-image during building (i.e., --build-arg="BASE_IMAGE=debian:trixie" for example). The default being debian:stable.

Other updates are:

• github workflow to build and test the standalone image at every push and pull request to master/main (to replace the travis script)
• inclusion of BASE_IMAGE in .env file for the docker-composes
• formating for better reading