fix docker nightly build

[TheGreatRefrigerator]

Is there an existing issue for this?

• I have searched the existing issues

Problem description

Currently, docker nightly images get built & published to dockerhub for basically every PR change, which consumes quite some quota.
This is neither a nightly build nor is it consistent, as the content of the image changes with the branch currently being built.

Proposed solution

Do normal nightly build if anything changed in main.
Similar to e.g. https://github.com/libgit2/libgit2/blob/main/.github/workflows/nightly.yml

Additional context

If we still want to do docker image builds for PRs we should do it differently

Forum Topic Link

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