66600f32dc introduced various improvements
to the determinism of the container image in this repository. This
change builds on this effort by introducing support for a container
image 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.
