This script is cheating, though!

Compared to the old WalletScrutiny script, it doesn't contain the actual build commands itself, but just downloads your Justfile script and runs that instead. I'd really like to be able to reproduce it with my own script, instead.
Yes, but the Justfiles are all open-source and verifiable themselves, they just greatly simplify things and remove the need to make changes to the WalletScrutiny script if the build environment or steps change.
That's true; and I have also read / 'verified' them myself, nothing 'fishy' going on, so I'm not mistrusting the Justfiles. Just trying to do it in a different way..

That's something to try, yes. The difference is simply due to me starting with the original podman-based script.
You would also need to replicate the build steps *exactly* from the
relevant Justfile.
Doing that would ensure that you're using the exact same environment. If you don't want to copy the files themselves, I would use them as a direct reference, as if there is even the smallest difference in the image you build under podman you can expect the hashes to not match.
That should already be the case, no? Steps themselves, as well as the order of execution, should be identical.