Docker: Update the time to pull docker images into agents

Signed-off-by: Benjamin Copeland <ben.copeland@linaro.org>
Change-Id: Ia102f15309a473d39463482d53425515b0d03acc
diff --git a/ci-dockerslaves-cleanup.yaml b/ci-dockerslaves-cleanup.yaml
index 0c7fe28..36f65be 100644
--- a/ci-dockerslaves-cleanup.yaml
+++ b/ci-dockerslaves-cleanup.yaml
@@ -24,7 +24,7 @@
     concurrent: false
     display-name: 'CI docker cleanup and image update'
     triggers:
-        - timed : '@daily'
+        - timed : '@hourly'
     axes:
         - axis:
             type: slave