CI: Schedule for branch docker doesn't work

[thetredev]

Seems like the feature doesn't even exist: https://github.com/orgs/community/discussions/16107

Keeping this issue open until maybe GitHub implements something we can use.

[thetredev]

Possible solution:

• Rename main branch to gh-pages and remove the empty pages branch
• Merge docker and gh-pages workflows and move them into a new main branch

This way, main's sole purpose would be to schedule GitHub workflows. It would need to git checkout the respective branch on a given schedule step.