Maintain a manifest file of all environment variables used by the image

I have been needing to maintain (and will build a tool to generate) a manifest of all environment variables used by the image, their defaults, category, and brief description when feasible. I'll either produce a json or yaml file that will be committed and baked into the image. I might add an `mc-image-helper` command to dump the file in various formats: json, yaml, CSV, markdown table, etc.

_Originally posted by @itzg in https://github.com/itzg/docker-minecraft-server/issues/1439#issuecomment-1554485684_

(This might be a duplicate and/or can supersede an existing issue with this description)

No files, tests, or entry points are named. Start by locating where the image's environment variables are defined and how image metadata is committed and baked in. Done would require a committed manifest containing variables, defaults, categories, and descriptions, with the optional output formats or mc-image-helper scope decided.

Written by the indexing model from the issue text.