`.ci/write-dockerfile.sh`: Move here from build/bin/

[mkoeppe]

... as suggested by @kwankyu in https://groups.google.com/g/sage-devel/c/dEa3i2Fn3ZY/m/nzUr2wc_AAAJ

To test: Commands such as tox -e docker-ubuntu-jammy-minimal -- config.status should still work.

:memo: Checklist

• [x] The title is concise and informative.
• [x] The description explains in detail what this PR is about.
• [x] I have linked a relevant issue or discussion.
• [ ] I have created tests covering the changes.
• [ ] I have updated the documentation and checked the documentation preview.

:hourglass: Dependencies

[github-actions[bot]]

Documentation preview for this PR (built with commit 97c3d22255e072f26e88d186012889fd11c31cbd; changes) is ready! :tada: This preview will update shortly after each push to this PR.

[kwankyu]

To test: Commands such as tox -e docker-ubuntu-jammy-minimal -- config.status should still work.

Yes, this works well.

[mkoeppe]

Thanks for the review!