dpkg: local cache for .deb files

With this patch, /var/cache/apt/archives directory content is preserved.
The directory is actually a bind mount of the
~/.cache/image-create/apt/$DISTRO_NAME directory, much like what we do
for ccache.
You can use DIB_APT_LOCAL_CACHE=0 to disable this behavior.

This trivial change improve performance A LOT (>30%), even if a local
HTTP proxy because:
 - there is no need to copy again and again the same files
 - we avoid network latency

The patch has been tested with Debian and Ubuntu with every elements
from the tripleo-image-elements repository, the final size of the cache
directory is about 700MB per distribution subdirectory.

Change-Id: I4fab499493f734c7c546d4d23b1a98f0e7523a39

elements/dpkg/README.md

@@ -7,6 +7,13 @@ can be used by any dpkg based element.
 
 The dpkg specific version of install-packages is also kept here.
 
-If an extra or updated apt key is needed then define DIB\_ADD\_APT\_KEYS with
-the path to a folder. Any key files inside will be added to the key ring before
-any apt-get commands take place.
+### Environment Variables ###
+
+* DIB\_ADD\_APT\_KEYS: If an extra or updated apt key is needed then define
+  DIB\_ADD\_APT\_KEYS with the path to a folder. Any key files inside will be
+  added to the key ring before any apt-get commands take place.
+* DIB\_APT\_LOCAL\_CACHE: You can use this variable to disable the internal cache
+  of the /var/cache/apt/archives directory by setting it to 0. The default is to bind
+  mount the ~/.cache/image-create/apt/$DISTRO_NAME directory in
+  /var/cache/apt/archives, this to cache the .deb files downloaded during the image
+  creation.

elements/dpkg/post-install.d/99-clean-up-cache

@@ -4,4 +4,6 @@
 set -eu
 set -o pipefail
 
-apt-get clean
+if ! mount | grep /var/cache/apt/archives; then
+    apt-get clean
+fi

elements/dpkg/root.d/99-shared_apt_cache

@@ -0,0 +1,16 @@
+#!/bin/bash
+
+set -eu
+set -o pipefail
+
+DIB_APT_LOCAL_CACHE=${DIB_APT_LOCAL_CACHE:-1}
+
+if [ $DIB_APT_LOCAL_CACHE = "0" ]; then
+    exit 0
+fi
+
+apt_cache_dir=$HOME/.cache/image-create/apt/$DISTRO_NAME
+if [ ! -d $apt_cache_dir ]; then
+    mkdir -p $apt_cache_dir
+fi
+sudo mount --bind $apt_cache_dir $TARGET_ROOT/var/cache/apt/archives