Use DIB_IMAGE_CACHE in dpkg element

The dpkg element was hardcoding ~/.cache/image-create as the cache path,
but elements should use DIB_IMAGE_CACHE as the root for the cache to
allow users to override the path. Replace the hardcoded path with one
that is rooted at $DIB_IMAGE_CACHE.

Change-Id: I76030654e7dfc7b7d4dfa87e082c14e7cee479b9

elements/dpkg/README.md

@@ -14,6 +14,6 @@ The dpkg specific version of install-packages is also kept here.
   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
+  mount the $DIB_IMAGE_CACHE/apt/$DISTRO_NAME directory in
   /var/cache/apt/archives, this to cache the .deb files downloaded during the image
   creation.

elements/dpkg/root.d/99-shared_apt_cache

@@ -9,7 +9,7 @@ if [ $DIB_APT_LOCAL_CACHE = "0" ]; then
     exit 0
 fi
 
-apt_cache_dir=$HOME/.cache/image-create/apt/$DISTRO_NAME
+apt_cache_dir=$DIB_IMAGE_CACHE/apt/$DISTRO_NAME
 if [ ! -d $apt_cache_dir ]; then
     mkdir -p $apt_cache_dir
 fi