There is only one rule, but it is important:
These are public images. Do NOT add any keys, certificates, passwords.
Create a Dockerfile in a new directory, say 'base/Dockerfile'.
Then do something like this:
./create core/base 3.20
In your project, use it as follows:
FROM voormedia/ruby-build:3.7 AS build
FROM voormedia/ruby:3.7