tests/docker: cleanup non-verbose output

Even with --quiet docker will spam the sha256 to the console. Avoid
this by redirecting stdout. While we are at it fix the name we echo
which was broken during 0b1a6490 (tests/docker: use direct RUNC call
to build containers).

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230829161528.2707696-3-alex.bennee@linaro.org>