Compliance with Deb822: Deprecated sources.list use .sources instead

[eshattow]

File: engine/install/debian.md

sources.list(5) says of the Deb822 ".sources" format "It is intended to make this format gradually the default format, deprecating the previously described one-line-style format, as it is easier to create, extend and modify for humans and machines alike..."

https://wiki.debian.org/DebianRepository/UseThirdParty

Note for clarity /etc/apt/keyrings as the Docker install guide suggest is in fact the correct location for a keyring installed by the system operator, so the guide gets it half-right already.

[docker-robott]

There hasn't been any activity on this issue for a long time.
If the problem is still relevant, mark the issue as fresh with a /remove-lifecycle stale comment.
If not, this issue will be closed in 14 days. This helps our maintainers focus on the active issues.

Prevent issues from auto-closing with a /lifecycle frozen comment.

/lifecycle stale

[eshattow]

/remove-lifecycle stale
/lifecycle frozen

[eshattow]

deb822 format will be the default for the next Debian release (either trixie or trixie+1) c.f. debuerreotype/debuerreotype#128

See comparable examples of how to form documentation for users:
https://wiki.winehq.org/Debian
https://qgis.org/en/site/forusers/alldownloads.html#debian-ubuntu