ci: add GitHub Actions script and Docker files

Adds a CI script to be run on push and pull requests. Eight cross-builds
are checked: (armhf, aarch64) x (make, cmake) x (Debian, Ubuntu).

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>