ci: tweak default docker image

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>

.buildkite/hooks/post-command

@@ -6,3 +6,7 @@
 # report disk usage:
 echo "--- $0 disk usage"
 df -h
+docker image prune --all --force
+df -h
+ls
+cat docker-images

.buildkite/hooks/pre-command

@@ -36,3 +36,7 @@ find -name index.lock -delete
 # return from where we started so we can find pipeline files from
 # git repo
 cd ${WORKDIR}
+
+# Save off what images was there before and clean things out
+docker images --filter=reference='zephyrprojectrtos/ci' --format "{{.Repository}}:{{.Tag}}" > docker-images
+docker image prune --all --force