ci: docker: remove source repos from fih-test

Update Dockerfile to only build a distro and required tooling, avoiding
adding source repositories, including tfm and mcuboot, so that it can be
reused without the need to rebuild all the time.

This should allow pushing the image under mcuboot/fih-test and
pulling+caching in Travis.

Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>