Fix overly zealous removal of APT state and cache files

Removal of the package index files, aka lists, is something that needs
to be done explicitly.  Doing so at `APT::Update::Post-Invoke` defeats
the purpose of running `apt-get update` in the first place.

scripts/docker-apt-clean

@@ -9,12 +9,10 @@
 # effect as running `apt-get clean` but circumvents the deadlock.
 
 DIR=/
-STATE=var/lib/apt/
 CACHE=var/cache/apt/
 
 eval $(apt-config shell DIR Dir)
-eval $(apt-config shell STATE Dir::State)
 eval $(apt-config shell CACHE Dir::Cache)
 
-find $DIR$STATE -type f -delete
-find $DIR$CACHE -type f -delete
+find $DIR$CACHE -type f -name '*.deb' -delete
+find $DIR$CACHE -type f -name '*.bin' -delete

scripts/remove-apt-lists

@@ -0,0 +1,17 @@
+#!/bin/sh
+# remove-apt-lists -- to keep Docker image size small
+# Copyright (C) 2017  Olaf Meeuwissen
+#
+# License: GPL-3.0+
+
+# There is no convenient Post-Invoke hook anywhere to hang this onto
+# but it is nice to be able to clean up the package index files when
+# you want to save space.
+
+DIR=/
+STATE=var/lib/apt/
+
+eval $(apt-config shell DIR Dir)
+eval $(apt-config shell STATE Dir::State)
+
+find $DIR$STATE -type f -name '*_dists_*' -delete