The rpxc-stretch branch of this repository has the changes to make the Docker image be based on Debian stretch. It also adds the dependencies needed by the gateway and addon-builder into the docker image.

This Dockerfile is based on: https://github.com/sdt/docker-raspberry-pi-cross-compiler

To update:

git checkout rpxc-stretch
./build.sh
./push.sh

If you use an account other that mozillaiot, then update https://github.com/mozilla-iot/addon-builder/blob/master/create-rpxc.sh and https://github.com/mozilla-iot/rpi-image-builder/blob/master/create-rpxc.sh to point to the Docker image that was pushed.