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.