engine: update gpg keyrings location for ubuntu/debian

[dvdksn]

Signed-off-by: David Karlsson 35727626+dvdksn@users.noreply.github.com

Proposed changes

• Changes the GPG keyrings location from /etc/apt/keyrings to /usr/share/keyrings
• Removes the install command since /usr/share/keyrings already exists by default

Related issues (optional)

Closes#17471

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[thaJeztah]

/cc @tianon PTAL

The location was updated to /etc/apt/keyrings in #14813, which mentions that that location was based on recommendations of the apt maintainers;

See https://tracker.debian.org/news/1305679/accepted-apt-240-source-into-unstable/:

* Install an empty /etc/apt/keyrings directory.
This directory is intended to provide an alternative to
/usr/share/keyrings for placing keys used with signed-by.

See also https://wiki.debian.org/DebianRepository/UseThirdParty?action=diff&rev2=47&rev1=46 (which was edited following a discussion with the APT maintainers about the expected usage):

If future updates to the key will be managed by an apt/dpkg package as recommended below, then it SHOULD be downloaded into /usr/share/keyrings using the same filename that will be provided by the package. If it will be managed locally , it SHOULD be downloaded into /etc/apt/keyrings instead.

FWIW; ISTR changing these instructions may potentially cause issues (package was signed by ...)

[thaJeztah]

leaving a "request changes" review, as I'd like to have more eyes on this change before we merge.

[tianon]

Yeah, the current documented path (/etc/apt/keyrings) is much more appropriate for user-managed keyrings like this one than the one suggested in the linked issue -- in a healthy deb-based system, files in /usr/share should typically only be put there by installed packages, which is why we should only use that path if we end up creating something like a docker-inc-keyring package that installs a keyring file there (and then we should use the exact same filename so that installing the package overwrites the manually created file).

For distribution versions which contain APT version 2.3.10 or newer, we could switch to the newer "deb822" format for configuring these sources, in which we can embed the key directly (see https://lists.debian.org/debian-devel/2021/11/msg00026.html), but from https://packages.debian.org/apt and https://packages.ubuntu.com/apt, that limits us to Debian bookworm+ (the latest stable just recently released) and Ubuntu jammy+ (which is 22.04, so slightly less new, but still pretty recent).

See also the following from the APT 2.4.0 changelog (https://tracker.debian.org/news/1305679/accepted-apt-240-source-into-unstable/):

 * Install an empty /etc/apt/keyrings directory.
This directory is intended to provide an alternative to
/usr/share/keyrings for placing keys used with signed-by.

[dvdksn]

Thank you so much for the detailed explanation @tianon. I'll close this PR and related issue.

Didn't know about the new APT features, let's see when we can incorporate those into our docs (probably when jammy/bookworm is our oldest supported version?)