misra-docker-images: Limit retention more by # of builds than by time

This job is run rarely, so keeping some history makes sense. Just keep
last 30 builds for a long time.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Ic1a7c1941aabb6631659b8723cca3b2554a6cae8
diff --git a/misra-docker-images.yaml b/misra-docker-images.yaml
index 5fdc69d..7cc02a9 100644
--- a/misra-docker-images.yaml
+++ b/misra-docker-images.yaml
@@ -8,8 +8,8 @@
                 - job-read
                 - job-extended-read
         - build-discarder:
-            days-to-keep: 90
-            num-to-keep: 200
+            days-to-keep: 500
+            num-to-keep: 30
     disabled: false
     node: build-amd64-private
     concurrent: false