docker: how to show the diffs between 2 images

Have a look at :

https://github.com/GoogleCloudPlatform/container-diff

This tool can diff local or remote docker images and can do so without requiring docker to be installed. It has file as well as package level "differs" (for example: apt, npm, and pip) so that you can more easily see the differences in packages that have changed between two docker images.

Disclaimer: I am a contributor to this project

I suppose you could send both images' file systems to tarballs via docker export CONTAINER_ID or docker save IMAGE_ID (updated based on comments)

Then use whatever tool you like to diff the file systems - Git, Rdiff, etc.