docker: base debian-tricore on qemu:debian9

We need both git and a working compiler to build the tools. Although
the qemu:debian9 image also has a bunch of extra dependencies it would
be fairly unusual for a user not to already have this layer available
for one of our many other docker images so lets not complicate things.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>