[ci,docker] Improve documentation for repo vars and build args

[cocosolos]

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• I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Adds some documentation for the repo vars added in #9014. Other minor comments and license stuff (docker/metadata-action handles some of this stuff already for CI published builds).

Steps to test these changes