Test out --squash option when building images

[nuest]

Take a loog at how the created image tarball looks - is squashing a plus for reproducibility?

[nuest]

https://stackoverflow.com/questions/41764336/how-does-the-new-docker-squash-work

see also example in README of https://github.com/goldmann/docker-squash

Squashing may improve manual inspection after a long time, because there are not multiple layers to explore. But it also reduces transparency as there is not one layer per Dockerfile instruction.