ci-dockerfiles-sanity-check: Add workspace-cleanup "publisher"

We're regularly hitting situation when 2 docker-related jobs are running
on the same EC2 builder, and 2nd fails due to out of disk space. Try
to alleviate this using this post-build action (which goes into "publishers"
department per Jenkins Job Builder).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I71114e11d3f9211748d4e5da556b38a514366606
diff --git a/ci-dockerfiles-sanity-check.yaml b/ci-dockerfiles-sanity-check.yaml
index 39c025b..23784a9 100644
--- a/ci-dockerfiles-sanity-check.yaml
+++ b/ci-dockerfiles-sanity-check.yaml
@@ -81,3 +81,4 @@
     publishers:
         - email:
             recipients: linaro-infrastructure-errors@lists.linaro.org
+        - workspace-cleanup