ci-dockerfiles-sanity-check: Reinstate matrix, use build-amd64-private

We can't convert marix job to normal job, that's apparently the reason
why there's as single-value matrix job in the first place.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Iecc9705c9585a9866b334f9cb2b1f2a321e44be6
1 file changed