Retry docker pull in tests if it fails. (#535)

test/run_tests.sh

@@ -1,5 +1,21 @@
 #!/bin/sh
 
+# Pull the builder image with retries if it doesn't already exist.
+retries=0
+builder_image=$(grep FROM test/Dockerfile | awk '{print $2}')
+
+if ! docker inspect "$builder_image"; then
+    until docker pull "$builder_image"; do
+        if [ retries -eq 3 ]; then
+            echo "Giving up downloading builder image, failing build."
+            exit 1
+        fi
+        echo "Docker pull failed, retrying."
+        ((retries++))
+        sleep 1
+    done
+fi
+
 docker build -f test/Dockerfile -t kudo-test .
 if [ $? -eq 0 ]
 then