Add test to the pipeline before pushing the image

[Morl99]

closes #21

[Morl99]

@jschirrmacher do you care to review this change? I have managed to find a way to test the image within the action before pushing it. We really only know if this works once we have merged, since there are steps in the workflow that only run on main.

[jschirrmacher]

So now you create and push a docker image in any case, just with different tags, right?
Apart from the number of images that are created, I have no concerns.

[Morl99]

Actually, the number of images (or rather layers) is unchanged, unless we actually have a faulty image. The retagged images are unchanged compared to the first one.

Thanks for looking over it. Using the registry was the only way possible, since docker cannot locally load multi architecture images built with buildx.