test: add Dockerfile
test: add Dockerfile

This will be used by the Docker Hub autobuild to build the rauc/testing
docker image that will be used for RAUC's Travis CI runs soon.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

ejoerns/rauc,rauc/rauc,jluebbe/rauc,ejoerns/rauc,jluebbe/rauc,ejoerns/rauc,jluebbe/rauc,rauc/rauc,rauc/rauc