gh-37841: `.ci/write-dockerfile.sh`: Move here from build/bin/
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... 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.
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URL: https://github.com/sagemath/sage/pull/37841
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe
