gh-38747: Fix build.yml broken for releases
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The workflow build.yml does not work well with releases:
https://github.com/sagemath/sage/actions/runs/11091397369
We fix it with
```yaml
- name: Get changed files and packages
id: changed-files
uses: tj-actions/changed-files@v45
with:
# File extensions for doctests per
sage.doctest.control.skipfile
files_yaml: |
configures:
- 'build/pkgs/*/spkg-configure.m4'
pkgs:
- 'build/pkgs/**'
- '!build/pkgs/_**'
- '!build/pkgs/configure/**'
- 'pkgs/**'
doctests:
- 'src/**/*.{py,pyx,pxd,pxi,sage,spyx,rst,tex}'
- '!src/{setup,conftest*}.py'
```
Note the two lines added
```yaml
- '!build/pkgs/_**'
- '!build/pkgs/configure/**'
```
Along the way, we make `.ci/write-dockerfile.sh` produce well formatted
`Dockerfile` to improve readability.
test:
https://github.com/kwankyu/sage/actions/runs/11122936897/job/30905299233
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preview.
### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/38747
Reported by: Kwankyu Lee
Reviewer(s): Matthias Köppe
