Disable pull of updated Shippable docker image.

This should improve reliability of our jobs on Shippable.

The Shippable AMIs should already have the latest docker image,
per Shippable support, so pull doesn't provide any benefit for us.

shippable.yml

@@ -58,6 +58,9 @@ build:
   pre_ci:
     - docker images | grep u14pyt
   pre_ci_boot:
+    image_name: drydock/u14pyt
+    image_tag: prod
+    pull: false
     options: "--privileged=false --net=bridge"
   ci:
     - test/utils/shippable/timing.sh test/utils/shippable/shippable.sh