clean-unit-test-docker: script to clean old images

Since Docker images created by build-unit-test-docker now have
a date-stamp and content hash as part of their tag, a system's
Docker image repository could grow quite large if not cleaned up.
Add a script that easily identifies old Docker images created by
that script and removes them (anything older than the current ISO
week).

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I316e71ba0b336035e33ebee223f7faf702eaeb2c
diff --git a/jenkins/run-build-script-ci b/jenkins/run-build-script-ci
index 3181b80..d49c9c5 100755
--- a/jenkins/run-build-script-ci
+++ b/jenkins/run-build-script-ci
@@ -13,4 +13,5 @@
 
 # Now run some of our most used CI scripts
 ./scripts/build-unit-test-docker
+./scripts/clean-unit-test-docker
 ./scripts/build-qemu-robot-docker.sh