Improve build speeds on release

[hendrikmaus]

All releases always start with cold caches as there is nothing created on merge to master that has the shared cache key it seems.

This could be fixed by extending the CI workflow. It is lacking a containerization simulation. So I propose to add it to the CI workflow, without pushing the image.

[hendrikmaus]

I created https://github.com/hendrikmaus/custom-cache-action in the meantime to enable build caching when working on tags.