docker: add debian/amd64 based on Stretch

By itself this doesn't add much to our coverage. However later patches
will extend this image to include more bleeding edge libraries which
are not yet widely available in distros.

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