66600f32dc introduced various improvements
to the determinism of the container image in this repository. This
change builds on this effort by ensuring that the base image is pulled
by digest. Image digests are immutable references, unlike tags, which
are mutable (except when optionally configured as immutable in certain
container registries, but not `docker.io`).
Make a major change to the `reproduce-image.py` script: drop `diffoci`,
build the container image, and ensure it has the exact same hash as the
source image.
We can drop the `diffoci` script when comparing the two images, because
we are now able build bit-for-bit reproducible images.
