Document referring to container images by their hash instead of their

[oddhack]

name, as an alternative to the tag system.

Minor updates to asciidoctor-spec image to work with the current REUSE and update the Node.js version to current (18).

@rpavlik this may be of interest to you - someone on my Facebook (?! :-)) suggested using hashes and I think that's preferable for the Vulkan CI. The trick for determining the hash came from

https://stackoverflow.com/questions/32046334/where-can-i-find-the-sha256-code-of-a-docker-image?noredirect=1&lq=1