> most docker images are instructions to (more or less) reproducibly create an image

I can't recall ever encountering such a thing myself. On the other hand, I've seen LOTS of this sort of thing:

  RUN apt-get update && \
    apt-get -y install apache2

The above is taken from official AWS documentation[1] but it's pretty rampant.

As I mentioned above, when the instructions are reproducible, there's no point using Docker to run them; a shell script would suffice.

[1] https://docs.aws.amazon.com/AmazonECS/latest/developerguide/...