Fix CI pipeline to handle different image names

(Documenting after the fact)

For every new release the image name changes, e.g. miranda-image, nostromo-image, orion-image... This led to two small annoyances:

1. For setting up CI build for every new baseline, a unique pipeline change was needed instead of using a template. It was eventually set by environment variable modifying a shared pipeline template, but that variable still needs to be set.

That is not too bad since it's done only once for each new pipeline (once every new release) and then never changes for that pipeline.

What's worse is:

2. GitHub Pull Requests could be aimed at different branches. We could not keep one single pipeline for PR checks with one single image-name, because then pull-requests aimed at a different branch (e.g. the previous one) would not build since the pipeline instruction tries to build the wrong image-name for that PR.

We handled problem 2 by having two different pipelines with different image names (current, and previous). But that will report one successful and one failing pipeline build for every PR...