scripts/docker-cleanup: buildkit cache too

In newer docker versions, a buildkit cache was introduced. It gets used
while building images. Clean it up as well.

Related: https://osmocom.org/projects/osmocom-servers/wiki/Docker_cache_clean_up
Change-Id: Icf5237def75d4bcef6b0065f3f1f1da2ff362322
1 file changed