Interesting... but also the "power" from container is precisely that the description itself, i.e. the Dockerfile, is sufficient. So I'm not sure what the main advantage of either providing the image itself via a registry or providing just the Dockerfile with the base image to another registry then adding steps add.

An image is immutable, many Dockerfiles are not idempotent. Running them multiple times at different times can lead to different results easily.

Building every time takes a lot of time. Both on download and execution time.

Also when building an image you may need to have access to resources that are unavailable where you actually want to run the image. So I see definitely reason for both to exist.

Or am I misunderstanding your point?