Update Ubuntu/Debian installation instructions to use Signed-By option (

#26906)

The use of `apt-key` to install has also been removed as it is now deprecated
and will be last available in Debian 11 and Ubuntu 22.04.

Also updates the Docker repository setup instructions in container-runtimes.md,
to now refer to the respective instructions at https://docs.docker.com/engine/install/
which has already made the move to use the signed-by option.