Dockerfiles look deterministic, but they are not

Apologies for editorializing the title a bit :-)

Reading through the comments in https://news.ycombinator.com/item?id=39720007 I saw a common misconception pop up again: Dockerfiles are not deterministic. But they _look_ like they are, and even are for a while: Build a Dockerfile on your local machine; then build it again => most likely exactly the same container. Stuff starts to break down quickly though; so I did this writeup some time ago that should be informative for the wider community.