containerfile: automatically search for distro docker files

This makes the container file element search the active element list
for `containerfiles/${DIB_RELEASE}` for building.  This makes it easy
to write wrappers for ubuntu/fedora/etc. containerfile elements.

Change-Id: I68f1d928e54a70bad76985ddd3e156bb5f978b0d

diskimage_builder/elements/containerfile/README.rst

@@ -5,5 +5,10 @@ containerfile
 Base element for creating images from container files (aka
 Dockerfiles).
 
-To use this element, create a container file (Dockerfile), and supply
-the path to that file in the environment variable `DIB_CONTAINER_FILE`.
+Usually this element will be called via a more specific distro element
+which provides an environment for building a full image.  This element
+will search active elements for a container file located in
+`containerfiles/${DIB_RELEASE}`.
+
+Alternatively, to use this element directly supply the path to a
+container file in the environment variable `DIB_CONTAINER_FILE`.

diskimage_builder/elements/containerfile/root.d/08-containerfile

@@ -25,7 +25,26 @@ if [ -f ${TARGET_ROOT}/.extra_settings ] ; then
     . ${TARGET_ROOT}/.extra_settings
 fi
 
-[ -n "$DIB_CONTAINER_FILE" ]
+if [ -z "${DIB_CONTAINER_FILE:-}" ]; then
+    _xtrace=$(set +o | grep xtrace)
+    set +o xtrace
+
+    eval declare -A image_elements=($(get_image_element_array))
+
+    for i in "${!image_elements[@]}"; do
+        element=$i
+        element_dir=${image_elements[$i]}
+
+        containerfile="${element_dir}/containerfiles/${DIB_RELEASE}"
+        if [ -f "${containerfile}" ]; then
+            echo "Found container file ${containerfile}"
+            DIB_CONTAINER_FILE="${containerfile}"
+            break
+        fi
+    done
+
+    $_xtrace
+fi
 
 # Use the image cache directory as the default context, so anything
 # there is automatically available for COPY commands.