Document how to persist 3rd party APT repositories together with their public key
Plan
In our doc about "Configuring additional APT repositories (for advanced users)" (https://tails.net/doc/persistent_storage/additional_software/index.en.html#index7h1), we should document how the public key can be included in the sources.list
file added for the 3rd-party repo (example).
Doc: https://manpages.debian.org/testing/apt/apt-key.8.en.html#DEPRECATION (see Signed-By
and "deb822-style sources.list").
Original report
Originally created by @goupille on #17510 (Redmine)
a user pointed out that in our documentation about “Configuring additional APT repositories” (https://tails.boum.org/doc/first_steps/additional_software/#index6h1), nothing is said about adding a key to apt’s keyring. so while it is possible to make persistent the addition of an unsigned repository to sources.list, it is not possible to do so with a signed one.
I’d say we should either recommand people to not permanently add whatever custom repo (i.e. not signed with a key already in apt’s keyring), or we should allow and document the persistence of apt’s keyring.