Keep "flab" out of the image

This integrates a custom script into the APT configuration.  That same
script is also run during the bootstrap.  A second custom script takes
care of the integration and removes itself when done.

Complements 13f84159.

bootstrap.sh

@@ -14,5 +14,9 @@ rootfs=/devuan/rootfs
 
 debootstrap --variant=minbase jessie $rootfs
 
-find $rootfs/var/cache/apt/ -type f -delete
-find $rootfs/var/lib/apt/ -type f -delete
+bindir=usr/local/bin
+cp -a scripts/* $rootfs/$bindir
+
+for script in $(ls $rootfs/$bindir); do
+    chroot $rootfs /$bindir/$script
+done

scripts/docker-apt-clean

@@ -0,0 +1,20 @@
+#!/bin/sh
+# docker-apt-clean -- to keep Docker image size small
+# Copyright (C) 2017  Olaf Meeuwissen
+#
+# License: GPL-3.0+
+
+# Running `apt-get clean` from within apt-get causes deadlock due to
+# the locking mechanism used by apt-get.  This script has a similar
+# 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

scripts/tweak-apt-config

@@ -0,0 +1,24 @@
+#!/bin/sh -eu
+# tweak-apt-config -- for use in a Docker image
+# Copyright (C) 2017  Olaf Meeuwissen
+#
+# License: GPL-3.0+
+
+DIR=/
+ETC=etc/apt
+PARTS=apt.conf.d
+
+eval $(apt-config shell DIR Dir)
+eval $(apt-config shell ETC Dir::Etc)
+eval $(apt-config shell PARTS Dir::Etc::Parts)
+
+cfg=$DIR$ETC$PARTS
+
+cmd=$(dirname $0)/docker-apt-clean
+cat > $cfg/docker-clean <<- EOF
+DPkg::Post-Invoke        { "if test -x $cmd; then $cmd; fi"; };
+APT::Update::Post-Invoke { "if test -x $cmd; then $cmd; fi"; };
+EOF
+chmod 0644 $cfg/docker-clean
+
+rm $0