Switch test to use alpine:3.5 while hub broken

There is a content trust issue with `alpine:latest` at present,
unblock the CI.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

alpine/base/test/bin/test.sh

@@ -5,7 +5,7 @@ set -ex
 docker version
 docker info
 docker ps
-DOCKER_CONTENT_TRUST=1 docker pull alpine
+DOCKER_CONTENT_TRUST=1 docker pull alpine:3.5
 docker run --rm alpine true
 docker pull armhf/alpine
 docker run --rm armhf/alpine uname -a