docker: rename debian stable -> 9 (Stretch)

We'll also want to support some older Debian combinations for
architectures that didn't make the Debian 9 cut.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
[AJB: extend commit msg]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>