shippable: pull images from registry instead of building

Now we have a source for registry images pull from there rather than
re-building every time on shippable.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Message-Id: <20200701135652.1366-41-alex.bennee@linaro.org>

.shippable.yml

@@ -26,12 +26,10 @@ env:
     - IMAGE=debian-ppc64el-cross
       TARGET_LIST=ppc64-softmmu,ppc64-linux-user,ppc64abi32-linux-user
 build:
-  pre_ci:
-    - make docker-image-${IMAGE} V=1
   pre_ci_boot:
-    image_name: qemu/${IMAGE}
+    image_name: registry.gitlab.com/qemu-project/qemu/${IMAGE}
     image_tag: latest
-    pull: false
+    pull: true
     options: "-e HOME=/root"
   ci:
     - unset CC