CI: Refresh the Debian 12 x86_64 container

CI: Refresh the Debian 12 x86_64 container

Rework the container to use heredocs for readability, and use
apt-get --no-install-recommends to keep the size down.

This reduces the size of the (uncompressed) container from 3.44GB to
1.97GB.

The container is left running the builds and tests as root.  A
subsequent patch will make the necessary changes to the test scripts
to allow test execution as a non-root user.

Signed-off-by: Javi Merino <javi.merino@cloud.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>