fix: un-hardcode repo+owner, sign commits via API

[JJGadgets]

Changelog

• Made repository owner and repository name an environment variable that reads from the GitHub Actions default environment variables, but can also be overridden if needed (override currently unused)
• Lowercase repository owner's username before using it in build-images.yaml, so that OCI pushes won't fail
• Use GitHub API to commit changes for both render-readme.yaml and Renovate so that commit signing is always done for all commits
• Added some descriptions & more detailed comments
• Made English easier for template-sensei

These changes have been tested on my own containers repo, so they should work on this repo too.