gh-36954: `tox -e update_docker_platforms`
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Furthering my hidden agenda of bringing portability tools to the masses.
- We add `.devcontainer/portability-*/devcontainer.json` files for all
platforms tested in our CI. This makes them selectable in GitHub
Codespaces.
- The files are generated by the new command `tox -e
update_docker_platforms`, which also updates the workflow file
`.github/workflows/docker.yml`.
- The command also [adds a table to the portability testing section in
the developer's guide](https://deploy-preview-36954--
sagemath.netlify.app/html/en/developer/portability_testing#using-our-
pre-built-docker-images-published-on-ghcr-io) with links to GitHub
Packages and friendly "Run in GitHub Codespaces" buttons.
- Because 'tis the season of Spielzeugneid, [we document how to run the
portability testing in GitHub Codespaces via Docker-in-
Docker](https://deploy-preview-36954--
sagemath.netlify.app/html/en/developer/portability_testing#testing-sage-
on-a-different-platform-using-docker), useful for those who cannot run
Docker on their own computers. This uses a new devcontainer config "tox-
docker-in-docker".
- We make various other updates to the portability testing section in
the developer's guide.
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### :hourglass: Dependencies
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- Depends on #37030 (split out from here)
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URL: https://github.com/sagemath/sage/pull/36954
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe, Tobias Diez
