add a live-build hook to clear apt cacahe of downloaded pakages form final image

cleanup is still incomplete https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=947123

config/hooks/normal/0900-clear-apt-cache.hook.chroot

@@ -0,0 +1,6 @@
+#!/bin/sh
+# clear APT package download cache to reduce the image size from ~100MB
+rm -vr /var/lib/apt/lists/
+
+# clear APT package list cache to reduce the image size from ~180MB
+rm -vr /var/cache/apt/